Add gcov code coverage to the test suite

scripts/coverage.py configures an instrumented build tree
(-DAKERR_COVERAGE=ON), runs the CTest suite in it, and reports merged
gcov line/branch/function coverage per library source. Like the mutation
harness it has no third-party dependencies and supports --threshold and
--junit; thresholds gate each file as well as the total so the generated
status-name table cannot mask a regression in src/error.c.

Only the library is instrumented. The public header's macros cannot be
measured this way -- GCC attributes an expanded macro to its call site,
so header logic would report as lines of the test that used it -- which
is what mutation testing against include/akerror.tmpl.h is for.

Coverage flags are applied per target rather than globally, so they do
not leak into the exported/installed target interface.

Current numbers for src/error.c are 94.0% line and 59.5% branch; the CI
gate is set to 90/50 to keep headroom, matching the convention used for
the mutation score threshold.

Tests run: ctest (23/23), cmake --build build --target coverage,
threshold gate verified failing at --threshold 99, cmake --install
checked for flag leakage, out-of-tree --build-dir checked.

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

test.sh

@@ -2,3 +2,4 @@
  cmake --build build
  ctest --test-dir build --output-on-failure --output-junit "$(pwd)/ctest-junit.xml"
  python3 scripts/mutation_test.py --target src/error.c --junit mutation-junit.xml --threshold 65
+ python3 scripts/coverage.py --junit coverage-junit.xml --threshold 90 --branch-threshold 50