ci, add ASan and UBSan jobs with separate badges

[steven-varga]

Summary

Add two separate CI jobs — one for AddressSanitizer, one for UndefinedBehaviorSanitizer — each emitting its own badge. Split as two jobs so README and staging-build-matrix.md can show independent pass/fail status.

Scope

Workflow

• Add asan job to .github/workflows/ci.yml:
  • runs-on: ubuntu-24.04, CMAKE_BUILD_TYPE=Debug
  • Compiler: clang-20 (matches matrix top-end)
  • Flags: -fsanitize=address -fno-omit-frame-pointer -O1 -g
  • ASAN_OPTIONS=detect_leaks=1:halt_on_error=1:abort_on_error=1:strict_string_checks=1
  • Runs ctest --output-on-failure. Failure → red.
• Add ubsan job to .github/workflows/ci.yml:
  • Same runner / compiler / build type.
  • Flags: -fsanitize=undefined -fno-sanitize-recover=undefined -fno-omit-frame-pointer -O1 -g
  • UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1
  • Runs ctest --output-on-failure.
• Each job emits its own badge JSON artifact (status-asan.json / status-ubsan.json), consumed by the existing generate-badges job.
• Reuse LLVM toolchain cache from issue ci, align CI matrix with h5cpp staging and replace HDF5 dep with LLVM toolchain #12's setup.

Badges

• Add [![ASan][asan-badge]] and [![UBSan][ubsan-badge]] rows to the top badge bar in README.md. Point at badges/asan.svg and badges/ubsan.svg.
• Add same badges to staging-build-matrix.md pointing at badges-staging/asan.svg and badges-staging/ubsan.svg.
• Update generate-badges job to also emit the two sanitizer SVGs into the badge output dir.

Acceptance criteria

• Two new jobs visible in Actions UI on every push.
• Two distinct badges in README badge bar.
• Two distinct badges in staging-build-matrix.md badge bar.
• Failing ASan or UBSan run flips the corresponding badge to red.

Dependencies

• Blocked by: ci, align CI matrix with h5cpp staging and replace HDF5 dep with LLVM toolchain #12 (CI must build), test, add transform fixture pack for coverage #13 (fixtures provide what to sanitize).

Conventions

• Branch: cut from staging, named <N>-ci-add-asan-and-ubsan-jobs.
• Commits: [#<N>]:svarga:ci, <description>.
• PR: targets staging, title [#<N>]:svarga:ci, add ASan and UBSan jobs with separate badges.