Add ON_CLOSE close-count commit support and update CI compiler matrix

[marcoSanti]

Summary

Added runtime support for committing ON_CLOSE files after a configured number of closes, and updates the CI/CD pipeline.

Close-count commit support

• Add Engine::increaseCloseCount(path), which records one logical close for an ON_CLOSE path and returns whether the path is committed after the call. It is also exposed in the Python bindings.
• Add MonitorInterface::increaseCloseCount(path, threshold), which atomically records a close and commits the path once the threshold is reached. Backends without a persistent counter return std::nullopt.
• FileSystemMonitor: close counts are persisted on the filesystem. Counts greater than one require CAPIO_METADATA_DIR to point to a trusted, per-run metadata directory that supports atomic exclusive create/unlink.
• MulticastMonitor: add the COUNT_SET and COUNT_GET message types. Each node keeps the highest count it has seen from each origin. These counts are volatile and eventually consistent.
• Monitor::increaseCloseCount records the close in every backend that supports counting.
• Document the new semantics in doxygen/semantics.md.
• Add tests/cpp/test_runtime_commit.hpp, with runtime configs for filesystem-only, multicast-only and mixed monitor setups.

CI/CD

• Drop gcc 9-11, whose official images are no longer maintained.
• Add gcc 14-16.
• Replace the silkeh/clang images with zhongruoyu/llvm-ports (trixie) and extend clang coverage to versions 13-23.
• Drop the macos-15-intel runner from the Python bindings workflow, since it is no longer supported. The README now shows macOS x86_64 as having no CI/CD support.
• Compute coverage inside the main test matrix (Debug, gcc:16) instead of in a separate rebuild job. The Codecov job now only downloads and uploads coverage.xml.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.74086% with 67 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.91%. Comparing base (1b63fb6) to head (e2ade71).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/monitors/FileSystem.cpp	65.83%	22 Missing and 19 partials ⚠️
src/monitors/Multicast.cpp	79.16%	6 Missing and 14 partials ⚠️
src/Engine.cpp	94.91%	1 Missing and 2 partials ⚠️
src/Monitor.cpp	86.36%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
- Coverage   95.40%   92.91%   -2.49%     
==========================================
  Files          19       19              
  Lines        1675     1948     +273     
  Branches     1218     1394     +176     
==========================================
+ Hits         1598     1810     +212     
- Misses         33       60      +27     
- Partials       44       78      +34     

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