Expand error status test coverage

- Added explicit status validation across error handling tests.
- Added lifecycle and slot-leak checks to older tests.
- Improved unhandled-error propagation coverage.
- Added repository guidance and a test runner script.
- Verified all 23 CTest tests pass.

Co-Authored by Codex GPT 5.4

test.sh

@@ -0,0 +1,4 @@
+ cmake -S . -B build
+ cmake --build build
+ ctest --test-dir build --output-on-failure --output-junit "$(pwd)/ctest-junit.xml"
+ python3 scripts/mutation_test.py --target src/error.c --junit mutation-junit.xml --threshold 65