Add code coverage to the CTest suite

New AKSL_COVERAGE option instruments the library and its tests with
--coverage -O0 and wires the report into the suite itself, so a plain
ctest --test-dir build-coverage both runs the tests and produces coverage.

Two CTest entries do the work, held in order by a CTest fixture rather
than by declaration order so they also hold under ctest -j:
coverage_reset (FIXTURES_SETUP) clears the .gcda counters before any test,
since gcov counts are cumulative and would otherwise fold in earlier runs;
coverage_report (FIXTURES_CLEANUP) aggregates gcov output afterwards.
AKSL_COVERAGE_THRESHOLD / AKSL_COVERAGE_BRANCH_THRESHOLD gate the report,
the same regression-ratchet idea as the mutation score. The `coverage`
target builds, runs and prints in one step.

scripts/coverage.py parses gcov's JSON output, aggregates line, branch and
function counts across translation units, and lists every uncovered line
and never-called function -- the actionable half, as with surviving
mutants. Python stdlib plus gcc's own gcov only: no lcov, gcovr or
genhtml. It also writes coverage-summary.txt (CTest hides the output of a
passing test) and a Cobertura coverage.xml for CI publishers.

Instrumentation is per target, so deps/libakerror stays out of the report.
The mutation harness now ignores build*/ and gcov artifacts when copying
the tree, so a coverage build does not slow it down.

Baseline on src/stdlib.c: 52.0% of lines, 23.6% of branches, 8 of 21
functions. The uncovered functions are the untested wrappers the mutation
survivors already point at (printf, ato*, stream, realpath, strhash).

Verified:
  cmake -S . -B build-coverage -DAKSL_COVERAGE=ON
  cmake --build build-coverage --target coverage      # 8/8, report printed
  ctest --test-dir build-coverage -j8                 # fixture order holds
  cmake -S . -B build-coverage -DAKSL_COVERAGE=ON -DAKSL_COVERAGE_THRESHOLD=60
  ctest --test-dir build-coverage --output-on-failure  # gate fails as expected
  ctest --test-dir build --output-on-failure           # 6/6, no .gcda emitted
  ctest --test-dir build-asan --output-on-failure      # 6/6
  scripts/mutation_test.py --target src/stdlib.c --list # 173 mutants, unchanged
Totals match gcov itself: 51.98% of 202 lines, 23.60% of 661 branches.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

AGENTS.md

@@ -8,7 +8,8 @@ structures in `libakerror` error contexts. Public API declarations live in
 one-file CTest executables under `tests/test_<name>.c`, with shared test helpers
 in `tests/aksl_capture.h`. CMake package templates are in `cmake/` and
 `akstdlib.pc.in`. The vendored dependency is `deps/libakerror`; update it as a
-submodule rather than editing generated files under `build/` or `build-asan/`.
+submodule rather than editing generated files under `build/`, `build-asan/` or
+`build-coverage/`.
 
 ## Build, Test, and Development Commands
 
@@ -29,6 +30,15 @@ cmake --build build-asan
 ctest --test-dir build-asan --output-on-failure
 ```
 
+For coverage, configure a third tree; the report is part of that suite (the
+`coverage_reset` / `coverage_report` CTest entries) and also lands in
+`build-coverage/coverage-summary.txt`:
+
+```sh
+cmake -S . -B build-coverage -DAKSL_COVERAGE=ON
+cmake --build build-coverage --target coverage
+```
+
 Run `cmake --build build --target mutation` only when you need the slower
 mutation harness. `rebuild.sh` installs to `/home/andrew/local` and removes the
 local build directory, so treat it as a local convenience script.