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>

README.md

@@ -269,3 +269,16 @@ PASS(e, aksl_atoi(width->data, &screenwidth));
 PASS(e, akgl_heap_release_string(width));
 ```
 
+
+## Mutation testing
+
+The mutation harness makes one deliberate source-code change at a time in a scratch copy, then rebuilds and runs the passing CTest suite to measure whether tests detect the change. The known-failing `character` test is excluded by default.
+
+```sh
+cmake --build build --target mutation
+scripts/mutation_test.py --target src/tilemap.c --list
+scripts/mutation_test.py --target src/tilemap.c --max-mutants 10
+scripts/mutation_test.py --threshold 40 --junit mutation-junit.xml
+```
+
+The default run covers all libakgl-owned files under `src/`. Use repeated `--target` options to narrow the scope. A surviving mutant identifies behavior that the current tests do not verify; the script prints its file, line, operator, and exact edit. The real working tree is never mutated.