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

src/typosaurus_sandbox/domain/__init__.py

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

src/typosaurus_sandbox/domain/calculator/__init__.py

@@ -0,0 +1,6 @@
+# retoor <retoor@molodetz.nl>
+
+from typosaurus_sandbox.domain.calculator.operations import add, clamp, clamp_to_byte, subtract, variance
+
+__all__ = ["add", "subtract", "clamp", "clamp_to_byte", "variance"]
+

src/typosaurus_sandbox/domain/calculator/operations.py

@@ -0,0 +1,33 @@
+# retoor <retoor@molodetz.nl>
+
+from typing import Sequence
+
+
+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))
+
+
+def variance(values: Sequence[float]) -> float:
+    if not values:
+        raise ValueError
+    mean = sum(values) / len(values)
+    return sum((x - mean) ** 2 for x in values) / len(values)
+