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>

scripts/mutation_test.py

@@ -277,8 +277,11 @@ def write_junit(path, records, targets):
 
 
 def copy_tree(src, dst):
-    ignore = shutil.ignore_patterns("build", ".git", "*.o", "*.so", "*~",
-                                    "#*#", "*.iso", "*.png")
+    # "build*" covers build/, build-asan/ and build-coverage/: the harness
+    # configures its own tree inside the copy, so copying those is pure IO --
+    # and a coverage tree drags along every .gcno/.gcda as well.
+    ignore = shutil.ignore_patterns("build*", ".git", "*.o", "*.so", "*~",
+                                    "#*#", "*.iso", "*.png", "*.gcno", "*.gcda")
     shutil.copytree(src, dst, ignore=ignore, symlinks=True)