ci: add -Dcoverage_tool=gcov|llvm option, wire native llvm-cov in matrix

[sublimator]

Summary

Adds a -Dcoverage_tool=gcov|llvm CMake option so we can use native LLVM source-based coverage (-fprofile-instr-generate -fcoverage-mapping → llvm-profdata → llvm-cov) as an alternative to the existing gcov + gcovr pipeline. Default stays gcov for backwards compat.

Motivation: gcov data is racy under parallel test runs (cf. #737 "suspicious gcov hit counts"). Native llvm-cov is faster, has better branch/region data, and doesn't share that failure mode.

CMake

• RippledSettings.cmake — new coverage_tool cache var (gcov | llvm) with validation
• RippledInterface.cmake — split coverage compile/link flags by tool via genex
• RippledCov.cmake — dispatch to the new helper when coverage_tool=llvm
• cmake/CodeCoverageLLVM.cmake (new) — setup_target_for_coverage_llvm() driving profraw → profdata → export with format-aware output (lcov / json / txt / html)

CI

• New clang-20 llvm-cov coverage matrix row (apt.llvm.org bootstrap for clang >= 19 since 24.04 default repos cap at clang-18)
• Conditional install of llvm-N vs gcovr based on coverage_tool
• Coverage artifact + Codecov upload generalised to coverage.lcov | coverage.xml; Codecov uploads tagged with flags: ${{ matrix.coverage_tool }} so the streams stay distinguishable
• Coverage cmake-args switched to *_FLAGS_DEBUG so the build action's -stdlib=libc++ flag isn't clobbered

TEMP — revert before merging to dev

• Matrix narrowed to just the new llvm-cov row on non-main refs only so we can iterate without burning runners on the rest
• coverage-llm added to the push trigger so direct pushes to the branch fire CI

Both temporary changes are guarded so they cannot affect dev/candidate/release runs, but they should still be reverted before merge.

Test plan

• CI run on this branch passes the new clang-20 llvm-cov coverage row end-to-end
• Codecov accepts the coverage.lcov upload (Codecov v5 with flags: llvm)
• Verify the produced coverage.lcov actually contains xrpld//libxrpl/ entries (not just empty / test-excluded coverage)
• Sanity-check report numbers vs. the gcc-13 gcovr report for major modules
• Revert the two temp items (matrix narrowing + push trigger entry) before flipping out of draft

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.40%. Comparing base (5e8d26f) to head (273273d).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #740      +/-   ##
==========================================
- Coverage   66.55%   66.40%   -0.16%     
==========================================
  Files         831      853      +22     
  Lines       78166    85020    +6854     
  Branches    44374    21321   -23053     
==========================================
+ Hits        52023    56457    +4434     
- Misses      17797    19820    +2023     
- Partials     8346     8743     +397     

Flag	Coverage Δ
llvm	66.40% <ø> (?)

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