Add mutation testing to validate the test suite

Introduce a self-contained mutation testing harness that verifies the unit
tests actually catch bugs: it makes small deliberate breakages to the library
(flip comparisons, delete statements, swap true/false, etc.), rebuilds, and
runs the whole CTest suite against each mutant. Tests that still pass reveal a
gap; tests that fail "kill" the mutant.

- scripts/mutation_test.py: the engine (stdlib only, no LLVM/clang deps).
  Operators ROR/LCR/BCR/AOR/ICR/SDL over src/error.c and the macro header.
  Mutates a scratch copy, never the working tree. Supports --target, --list,
  --max-mutants sampling, --threshold gating, --timeout.
- CMakeLists.txt: 'mutation' custom target (cmake --build build --target mutation).
- .gitea/workflows/ci.yaml: gated mutation job on src/error.c (threshold 65%).
- tests/MUTATION.md: how to run, interpret survivors, and known equivalents.

Close the real gaps the harness found in src/error.c (score 53% -> 71%):
- err_error_names: the AKERR_* codes have their names registered by akerr_init
- err_release_clears: releasing a context wipes it before reuse
- err_pool_exhaust: akerr_next_error returns NULL when the pool is full and
  always hands back the lowest free slot

Also surfaced (documented, not fixed): AKERR_MAX_ERR_VALUE (+15) is below
AKERR_NOT_IMPLEMENTED (+16) and AKERR_BADEXC (+17), so those codes can never
have a name registered.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -21,3 +21,22 @@ jobs:
           cmake --install build
           cmake --build build --target test
       - run: echo "🍏 This job's status is ${{ job.status }}."
+
+  mutation_test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository code
+        uses: actions/checkout@v4
+      - name: dependencies
+        run: |
+          sudo apt-get update -y
+          sudo apt-get install -y cmake gcc moreutils python3
+      # Verify the tests actually catch bugs: break the library many ways and
+      # confirm the suite fails. Gated on src/error.c (fast, deterministic).
+      # The threshold keeps headroom below the current score for equivalent
+      # mutants; see tests/MUTATION.md. Run the full default target locally for
+      # deeper (slower) coverage including the macro header.
+      - name: mutation testing
+        run: |
+          python3 scripts/mutation_test.py --target src/error.c --threshold 65
+      - run: echo "🍏 This job's status is ${{ job.status }}."