fix(mcp): report freshness with the indexer's own mtime source (#1714)

[umi008]

What does this PR do?

check_index_coverage reported freshness: "metadata_changed" for every file on Windows, always — including files checked seconds after a successful full re-index. The freshness reader recomputed mtime_ns from struct stat (which on Windows truncates to seconds via st_mtime), while the indexer records the FILETIME-derived nanosecond value from cbm_path_info_utf8. A byte-identical file therefore never matched, so the freshness field carried no signal and agents following the documented guidance were told to distrust a graph that was actually current.

Both readers now compare against the same source the writer uses:

• coverage_path_freshness (mcp.c) derives the comparison mtime/size from cbm_path_info_utf8 — the exact function the indexer records from — instead of a platform-dependent stat conversion. The Windows-only st_mtime truncation path is gone.
• The same truncation existed in the pipeline's incremental change-detection (pipeline_incremental.c), where it would mark every file changed on Windows; it now uses cbm_path_info_utf8 too, on all platforms.

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)

[github-actions]

Thanks for opening this — it has been seen, and it is queued.

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Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

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If this fixes a bug, a reproduction we can run is worth more than a description of the symptom.

Thanks for contributing, and sorry in advance for the wait.