score/time: Migrate coverage pipeline to LLVM source-based coverage (linux)

[lavrovvalera]

What

Migrate the score/time coverage pipeline from GCC/gcov to LLVM source-based
coverage (llvm-cov) for Linux builds. Keep the gcov pipeline for QNX.

Deliverables:

• quality/coverage/coverage.bazelrc — LLVM settings for Linux + gcov for QNX
• quality/coverage/llvm_cov/merger.py — per-test profraw→profdata zip generator
• quality/coverage/llvm_cov/reporter.py — final HTML + LCOV report generator
• quality/coverage/llvm_cov/reporter_wrapper.bzl — Bazel wrapper
• .bazelrc — imports quality/coverage/coverage.bazelrc
• MODULE.bazel — exposes @llvm_toolchain_llvm for llvm-cxxfilt
• .github/workflows/code-coverage.yml — custom workflow using new pipeline
• New time-x86_64-linux-coverage config for coverage builds (no GCC platform pin)

Why

GCC/gcov instruments at assembly level and generates phantom branches for:

• Exception-unwind edges on every non-noexcept call (affects ptp headers,
  std::make_shared in stub backends)
• GMock internal bookkeeping branches (affects all mock files)
• Assertion-abort branches that are only coverable via subprocess death tests

These phantom branches cause all mock files and ptp headers to show 50% branch
coverage permanently, making the 90% branch coverage goal of #47 unreachable
with gcov.

LLVM source-based coverage tracks source regions, not assembly branches:

• ptp headers: 100% branch (was 50% with gcov) ✅
• GMock mock files: 100% branch (was 50% with gcov) ✅
• No phantom exception-unwind branches ✅
• Baseline: 95.1% line, branch gaps only from untested code paths

References

• Improvement: Increase unit test coverage to 90% line + branch: score/time/ API #44 (covered: score/time tests)
• Tooling from eclipse-score/communication module (quality/coverage/llvm_cov/)

Acceptance Criteria

• bazel coverage --config=time-x86_64-linux-coverage //score/time/...
  completes without error
• lcov_report/lcov.dat is present in the reporter output zip and
  parseable as LCOV text
• CI workflow code-coverage.yml runs successfully on Linux x86_64
• All mock files and ptp headers show ≥ 90% line and ≥ 90% branch
• QNX coverage still works via --config=qnx with the gcov pipeline
• enable_llvm_coverage_for_death_tests enabled once score_cpp_policies
  is updated to ≥ 0.0.2 (death-test subprocess coverage)