feat: add call stack tracking and enhanced error reporting with file:line context

Implement a full call stack system that tracks function calls during interpretation, including native function boundaries. Add `push_call_frame`/`pop_call_frame` functions, a `CallStack` struct with depth-limited frames, and `print_stacktrace` for formatted error output. Introduce `error_with_context` to replace bare `error()` calls, and modify the tokenizer to track line/column/filename per token. Update the parser and interpreter to push/pop frames on function entry/exit, and add new Makefile targets (`demo-error-*`) plus test scripts (`test_error_stacktrace.rc`, `test_error_native.rc`, `test_error_undefined_var.rc`) and an `examples/error_handling_demo.rc` to demonstrate the new error reporting capabilities.

Makefile

@@ -19,6 +19,7 @@ SOURCES = $(SRC_DIR)/main.c \
           $(SRC_DIR)/string_utils.c \
           $(SRC_DIR)/preprocessor.c \
           $(SRC_DIR)/oop.c \
+          $(SRC_DIR)/error.c \
           $(SRC_DIR)/stdlib/stdlib.c \
           $(SRC_DIR)/stdlib/string/string_stdlib.c \
           $(SRC_DIR)/stdlib/math/math_stdlib.c \
@@ -136,6 +137,20 @@ test-eval: $(TARGET)
 test-validation: $(TARGET)
 	$(TARGET) $(TEST_DIR)/test_input_validation.rc
 
+demo-error-stacktrace: $(TARGET)
+	@echo "=== Demonstrating Full Stack Trace ==="
+	-$(TARGET) $(TEST_DIR)/test_error_stacktrace.rc
+
+demo-error-undefined: $(TARGET)
+	@echo "=== Demonstrating Undefined Variable Error ==="
+	-$(TARGET) $(TEST_DIR)/test_error_undefined_var.rc
+
+demo-error-native: $(TARGET)
+	@echo "=== Demonstrating Error in Native Function Call ==="
+	-$(TARGET) $(TEST_DIR)/test_error_native.rc
+
+demo-errors: demo-error-stacktrace demo-error-undefined demo-error-native
+
 run-http-simple: $(TARGET)
 	$(TARGET) $(EXAMPLE_DIR)/http_simple.rc
 
@@ -165,6 +180,14 @@ help:
 	@echo "  make test-async       - Async stdlib (20+ tests)"
 	@echo "  make test-oop         - OOP features (31 tests)"
 	@echo "  make test-preprocessor - Preprocessor (2 tests)"
+	@echo "  make test-eval        - Eval function (30 tests)"
+	@echo "  make test-validation  - Input validation (77 tests)"
+	@echo ""
+	@echo "Error Handling Demonstrations:"
+	@echo "  make demo-errors              - Run all error handling demos"
+	@echo "  make demo-error-stacktrace    - Show full stack trace with file:line"
+	@echo "  make demo-error-undefined     - Show undefined variable error with tips"
+	@echo "  make demo-error-native        - Show error during native function call"
 	@echo ""
 	@echo "Examples:"
 	@echo "  make run-http-simple            - Run http_simple.rc (single connection HTTP server)"
@@ -178,6 +201,7 @@ help:
 	@echo "    stdlib/math/    - Math functions"
 	@echo "    stdlib/io/      - I/O functions"
 	@echo "    stdlib/async/   - Async functions"
+	@echo "    stdlib/eval/    - Eval function"
 	@echo "  tests/            - Test programs (unittest framework)"
 	@echo "  examples/         - Example programs (.rc files)"
 	@echo "  build/            - Compiled object files"