Use Bazel for the sanitizer CI jobs

[philipcraig]

Closes #292.

The sanitizer CI jobs now build and test with Bazel instead of CMake. The branch first ran both side by side to compare them; the findings are below.

• .bazelrc gains asan, ubsan, and tsan configs with the same flags cmake/sanitizers.cmake applies. They use --copt so googletest is instrumented too, and --compilation_mode=dbg so reports carry symbolised stack traces.
• scripts/bazel.sh takes --asan, --ubsan, and --tsan, mirroring scripts/cmake.sh; --asan with --tsan is rejected. The CMake flags stay for local use.
• sanitizers.yml sets up Bazel with a per-sanitizer disk cache and runs ./scripts/bazel.sh --asan --ubsan and ./scripts/bazel.sh --tsan. Its paths filters track the Bazel inputs instead of the CMake ones.
• The change-detection skip moves from job level to step level in cmake.yml, bazel.yml, and sanitizers.yml, so skipped matrix jobs no longer report under the unexpanded name ${{ matrix.settings.name }} ${{ matrix.configuration }}.
• .bazelignore also ignores .claude, where Claude Code keeps agent worktrees whose build/ directories otherwise make bazel test //... fail to load.
• Docs updated.

The asan, tsan, and sanitizers check names are unchanged, so the required-check ruleset needs no change.

Findings from four runs with both build systems

Build-and-test step, seconds:

Run	cmake asan	cmake tsan	bazel asan	bazel tsan
Cold cache	85	78	91	92
Warm rerun, no change	88	78	21	16
Push after main changed protocol.hh	88	78	61	64
After rebase	89	68	77	65

• Bazel is never slower than CMake and drops to about 20 s when nothing changed. Whole-job times are equal within noise because the "Install GCC trunk" step (81 to 149 s, mostly the package's mkheaders postinst) dominates both; Cache the installed GCC trunk tree in CI #297 tracks caching it.
• The setup-bazel disk cache saves only on an exact key miss, so it froze at the first run's contents and later runs stayed partially warm; Save the Bazel disk cache on every CI run #298 tracks saving it per run.

[jbcoe]

Thanks for this. Everything will be a little bit faster to do now.