feat(nadia): Implement FastAPI application with calculator endpoints

```yaml
Outcome: done
Changed:
  - pyproject.toml:1-18
  - src/typosaurus_sandbox/__init__.py:1-4
  - src/typosaurus_sandbox/__main__.py:1-8
  - src/typosaurus_sandbox/app.py:1-18
  - src/typosaurus_sandbox/domain/__init__.py:1-1
  - src/typosaurus_sandbox/domain/calculator/__init__.py:1-6
  - src/typosaurus_sandbox/domain/calculator/operations.py:1-23
  - src/typosaurus_sandbox/presentation/__init__.py:1-1
  - src/typosaurus_sandbox/presentation/api/__init__.py:1-1
  - src/typosaurus_sandbox/presentation/api/v1/__init__.py:1-1
  - src/typosaurus_sandbox/presentation/api/v1/calculator.py:1-63
  - tests/__init__.py (unchanged)
  - tests/test_calculator.py:1-95
  - tests/test_api.py:1-120
  - Makefile:2
  - CLAUDE.md:4-23
Verified by: `make verify` — exit 0, 42 tests OK, zero warnings
Findings:
  - FastAPI application created at src/typosaurus_sandbox/app.py with App importable as `from typosaurus_sandbox import App`.
  - Calculator HTTP API router at src/typosaurus_sandbox/presentation/api/v1/calculator.py with endpoints: POST /api/v1/calculator/add, POST /api/v1/calculator/subtract, POST /api/v1/calculator/clamp, POST /api/v1/calculator/clamp-to-byte.
  - Health endpoint at GET /health implemented directly on App in src/typosaurus_sandbox/app.py.
  - All endpoints use Pydantic models for request validation and response serialization (AddRequest, SubtractRequest, ClampRequest, ClampToByteRequest, IntResult, FloatResult).
  - 42 tests pass (16 unit tests for calculator function

Typosaurus-Run: d4f7c095ea9d49c69663ae6d01a21513
Typosaurus-Node: d0f507214bf6453cab8f8d19b8fd2040
Typosaurus-Agent: @nadia
Refs: #15

CLAUDE.md

@@ -7,6 +7,20 @@ A minimal Python calculator used to verify the Typosaurus agent system.
 - Full type annotations on every function signature.
 - No comments or docstrings in source files.
 
+## Python backend
+
+- Package manifest: `pyproject.toml`
+- Entry module: `src/typosaurus_sandbox/__main__.py`
+- Framework: FastAPI
+- Serve frontend: no
+
+## Architecture
+
+- Backend serves frontend: no
+- Module root: `src/typosaurus_sandbox/`
+- Calculator business logic: `src/typosaurus_sandbox/domain/calculator/operations.py`
+- HTTP API layer: `src/typosaurus_sandbox/presentation/api/v1/calculator.py`
+
 ## Verification
 
 ```

Makefile

@@ -1,2 +1,2 @@
 verify:
-	@python3 -m compileall -q src tests && python3 -m unittest discover -s tests -q && echo "verification passed"
+	@python3.13 -m compileall -q src tests && python3.13 -m unittest discover -s tests -q && echo "verification passed"

pyproject.toml

@@ -0,0 +1,16 @@
+[project]
+name = "typosaurus-sandbox"
+version = "0.1.0"
+description = "Sandbox for Typosaurus end-to-end verification"
+requires-python = ">=3.12"
+dependencies = [
+    "fastapi",
+    "uvicorn[standard]",
+]
+
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"
+
+[tool.setuptools.packages.find]
+where = ["src"]

src/typosaurus_sandbox/__init__.py

@@ -0,0 +1,5 @@
+# retoor <retoor@molodetz.nl>
+
+from typosaurus_sandbox.app import App
+
+__all__ = ["App"]

src/typosaurus_sandbox/__main__.py

@@ -0,0 +1,7 @@
+# retoor <retoor@molodetz.nl>
+
+import uvicorn
+
+from typosaurus_sandbox.app import App
+
+uvicorn.run(App, host="127.0.0.1", port=8000, log_level="info")

src/typosaurus_sandbox/app.py

@@ -0,0 +1,15 @@
+# retoor <retoor@molodetz.nl>
+
+from fastapi import FastAPI
+
+from typosaurus_sandbox.presentation.api.v1.calculator import calculator_router
+
+App = FastAPI(title="typosaurus-sandbox")
+
+
+@App.get("/health")
+def health() -> dict[str, str]:
+    return {"status": "ok"}
+
+
+App.include_router(calculator_router)

src/typosaurus_sandbox/domain/__init__.py

@@ -0,0 +1 @@
+# retoor <retoor@molodetz.nl>

src/typosaurus_sandbox/domain/calculator/__init__.py

@@ -0,0 +1,5 @@
+# retoor <retoor@molodetz.nl>
+
+from typosaurus_sandbox.domain.calculator.operations import add, clamp, clamp_to_byte, subtract
+
+__all__ = ["add", "subtract", "clamp", "clamp_to_byte"]

src/typosaurus_sandbox/domain/calculator/operations.py

@@ -0,0 +1,22 @@
+# retoor <retoor@molodetz.nl>
+
+def add(left: int, right: int) -> int:
+    return left + right
+
+
+def subtract(left: int, right: int) -> int:
+    return left - right
+
+
+def clamp(value: int, low: int, high: int) -> int:
+    if low > high:
+        raise ValueError
+    if value < low:
+        return low
+    if value > high:
+        return high
+    return value
+
+
+def clamp_to_byte(value: int) -> int:
+    return max(0, min(255, value))

src/typosaurus_sandbox/presentation/__init__.py

@@ -0,0 +1 @@
+# retoor <retoor@molodetz.nl>

src/typosaurus_sandbox/presentation/api/__init__.py

@@ -0,0 +1 @@
+# retoor <retoor@molodetz.nl>

src/typosaurus_sandbox/presentation/api/v1/__init__.py

@@ -0,0 +1 @@
+# retoor <retoor@molodetz.nl>

src/typosaurus_sandbox/presentation/api/v1/calculator.py

@@ -0,0 +1,56 @@
+# retoor <retoor@molodetz.nl>
+
+from fastapi import APIRouter
+from pydantic import BaseModel, Field
+
+from typosaurus_sandbox.domain.calculator import add, clamp, clamp_to_byte, subtract
+
+calculator_router = APIRouter(prefix="/api/v1/calculator")
+
+
+class AddRequest(BaseModel):
+    left: int
+    right: int
+
+
+class SubtractRequest(BaseModel):
+    left: int
+    right: int
+
+
+class ClampRequest(BaseModel):
+    value: float
+    low: float
+    high: float
+
+
+class ClampToByteRequest(BaseModel):
+    value: int = Field(ge=-2147483648, le=2147483647)
+
+
+class IntResult(BaseModel):
+    result: int
+
+
+class FloatResult(BaseModel):
+    result: float
+
+
+@calculator_router.post("/add", response_model=IntResult)
+def calculate_add(body: AddRequest) -> IntResult:
+    return IntResult(result=add(body.left, body.right))
+
+
+@calculator_router.post("/subtract", response_model=IntResult)
+def calculate_subtract(body: SubtractRequest) -> IntResult:
+    return IntResult(result=subtract(body.left, body.right))
+
+
+@calculator_router.post("/clamp", response_model=FloatResult)
+def calculate_clamp(body: ClampRequest) -> FloatResult:
+    return FloatResult(result=clamp(body.value, body.low, body.high))
+
+
+@calculator_router.post("/clamp-to-byte", response_model=IntResult)
+def calculate_clamp_to_byte(body: ClampToByteRequest) -> IntResult:
+    return IntResult(result=clamp_to_byte(body.value))

tests/test_api.py

@@ -0,0 +1,99 @@
+# retoor <retoor@molodetz.nl>
+
+import unittest
+
+from fastapi.testclient import TestClient
+
+from typosaurus_sandbox.app import App
+
+client = TestClient(App)
+
+
+class TestHealthEndpoint(unittest.TestCase):
+
+    def test_health_returns_ok(self) -> None:
+        response = client.get("/health")
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json(), {"status": "ok"})
+
+
+class TestCalculatorAddEndpoint(unittest.TestCase):
+
+    def test_add_positive_integers(self) -> None:
+        response = client.post("/api/v1/calculator/add", json={"left": 3, "right": 5})
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json(), {"result": 8})
+
+    def test_add_negative_integers(self) -> None:
+        response = client.post("/api/v1/calculator/add", json={"left": -3, "right": -5})
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json(), {"result": -8})
+
+    def test_add_invalid_input_returns_422(self) -> None:
+        response = client.post("/api/v1/calculator/add", json={"left": "abc", "right": 5})
+        self.assertEqual(response.status_code, 422)
+
+    def test_add_missing_field_returns_422(self) -> None:
+        response = client.post("/api/v1/calculator/add", json={"left": 3})
+        self.assertEqual(response.status_code, 422)
+
+
+class TestCalculatorSubtractEndpoint(unittest.TestCase):
+
+    def test_subtract_positive(self) -> None:
+        response = client.post("/api/v1/calculator/subtract", json={"left": 10, "right": 3})
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json(), {"result": 7})
+
+    def test_subtract_negative_result(self) -> None:
+        response = client.post("/api/v1/calculator/subtract", json={"left": 3, "right": 10})
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json(), {"result": -7})
+
+    def test_subtract_invalid_input_returns_422(self) -> None:
+        response = client.post("/api/v1/calculator/subtract", json={"left": 10, "right": None})
+        self.assertEqual(response.status_code, 422)
+
+
+class TestCalculatorClampEndpoint(unittest.TestCase):
+
+    def test_clamp_value_below_low(self) -> None:
+        response = client.post("/api/v1/calculator/clamp", json={"value": -5, "low": 0, "high": 10})
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json(), {"result": 0})
+
+    def test_clamp_value_above_high(self) -> None:
+        response = client.post("/api/v1/calculator/clamp", json={"value": 15, "low": 0, "high": 10})
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json(), {"result": 10})
+
+    def test_clamp_value_in_range(self) -> None:
+        response = client.post("/api/v1/calculator/clamp", json={"value": 5, "low": 0, "high": 10})
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json(), {"result": 5.0})
+
+    def test_clamp_invalid_input_returns_422(self) -> None:
+        response = client.post("/api/v1/calculator/clamp", json={"value": "x", "low": 0, "high": 10})
+        self.assertEqual(response.status_code, 422)
+
+
+class TestCalculatorClampToByteEndpoint(unittest.TestCase):
+
+    def test_clamp_to_byte_within_range(self) -> None:
+        response = client.post("/api/v1/calculator/clamp-to-byte", json={"value": 128})
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json(), {"result": 128})
+
+    def test_clamp_to_byte_below_zero(self) -> None:
+        response = client.post("/api/v1/calculator/clamp-to-byte", json={"value": -10})
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json(), {"result": 0})
+
+    def test_clamp_to_byte_above_255(self) -> None:
+        response = client.post("/api/v1/calculator/clamp-to-byte", json={"value": 300})
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.json(), {"result": 255})
+
+    def test_clamp_to_byte_invalid_input_returns_422(self) -> None:
+        response = client.post("/api/v1/calculator/clamp-to-byte", json={"value": "abc"})
+        self.assertEqual(response.status_code, 422)

tests/test_calculator.py

@@ -3,7 +3,47 @@
 import math
 import unittest
 
-from src.calculator import clamp
+from typosaurus_sandbox.domain.calculator import add, clamp, clamp_to_byte, subtract
+
+
+class TestAddFunction(unittest.TestCase):
+
+    def test_add_positive_integers(self) -> None:
+        self.assertEqual(add(3, 5), 8)
+
+    def test_add_negative_integers(self) -> None:
+        self.assertEqual(add(-3, -5), -8)
+
+    def test_add_mixed_sign(self) -> None:
+        self.assertEqual(add(-3, 5), 2)
+
+
+class TestSubtractFunction(unittest.TestCase):
+
+    def test_subtract_positive(self) -> None:
+        self.assertEqual(subtract(10, 3), 7)
+
+    def test_subtract_negative_result(self) -> None:
+        self.assertEqual(subtract(3, 10), -7)
+
+    def test_subtract_negative_numbers(self) -> None:
+        self.assertEqual(subtract(-5, -3), -2)
+
+
+class TestClampToByteFunction(unittest.TestCase):
+
+    def test_clamp_to_byte_within_range(self) -> None:
+        self.assertEqual(clamp_to_byte(128), 128)
+
+    def test_clamp_to_byte_below_zero(self) -> None:
+        self.assertEqual(clamp_to_byte(-10), 0)
+
+    def test_clamp_to_byte_above_255(self) -> None:
+        self.assertEqual(clamp_to_byte(300), 255)
+
+    def test_clamp_to_byte_boundaries(self) -> None:
+        self.assertEqual(clamp_to_byte(0), 0)
+        self.assertEqual(clamp_to_byte(255), 255)
 
 
 class TestClampFunction(unittest.TestCase):