Add mutation testing harness

Port the scratch-copy mutation runner used by libakerror and libakstdlib, covering libakgl source files with sampling, thresholds, JUnit reports, and test timeouts. Add a CMake target and documentation, preserve the intentionally failing character test exclusion, and migrate the sprite test to the renderer backend so mutation baselines are green.

Co-authored-by: Codex (GPT-5) <noreply@openai.com>
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

AGENTS.md

@@ -26,6 +26,8 @@ ctest --test-dir build --output-on-failure
 
 Run one test while iterating, for example `ctest --test-dir build -R sprite --output-on-failure`. The `rebuild.sh` script also installs into a developer-specific `/home/andrew/local` prefix and removes existing outputs; prefer the portable commands above unless that exact workflow is intended.
 
+Run mutation testing with `cmake --build build --target mutation`. For a quick smoke run, use `scripts/mutation_test.py --target src/tilemap.c --max-mutants 10`; the harness mutates only a scratch copy and excludes the intentionally failing character test.
+
 ## Coding Style & Naming Conventions
 
 Follow the surrounding C style: braces on the next line for function bodies, spaces inside control-flow parentheses, and short, focused functions. Preserve the indentation of the file being edited; older files contain both spaces and tabs. Public symbols use the `akgl_` prefix, public types use `akgl_TypeName`, and constants/macros use `AKGL_UPPER_SNAKE_CASE`. Name matching header/source pairs by feature, such as `include/akgl/sprite.h` and `src/sprite.c`. No repository-wide formatter or linter is configured, so avoid unrelated formatting churn.