perf(desktop): cut startup log-directory scans from six to four

[simpleqt]

Fixes the startup directory-scan part of #584.

What

Every LogFileSink startup walks the log directory via ownedFiles() (a full readdirSync plus one lstatSync per owned log). The boot sequence in main.ts (new LogFileSink → enforceDirectoryCap() → purgeOlderThan(7) → writeHeader()) used to trigger six such walks:

1. constructor measureDirectoryBytes()
2. enforceDirectoryCap() ownedFiles()
3. purgeOlderThan ownedFiles()
4. purgeOlderThan closing measureDirectoryBytes()
5. first rollDate → loadState(all)
6. first rollDate → loadState(error)

(#584 counted ~5; the per-channel loadState pair makes it six.)

Change

• rollDate() fuses the two per-channel loadState scans into one ownedFiles() pass (resumeSegmentState helper keeps the per-entry matching logic byte-for-byte, including the equal-segment readdir-order tie behavior).
• purgeOlderThan() tracks surviving bytes inside the deletion loop instead of re-measuring the directory afterwards; locked old files still count toward the total, matching what the previous re-scan would have produced.

Boot now scans four times, and each daily date rollover scans once instead of twice. The per-line appendFileSync is untouched — closing after every append is the deliberate crash-safety tradeoff already documented in the code.

Verification

• vitest run tests/log-files.spec.ts — 15/15, including two new regression tests:
  • resumes rotation from pre-existing segments for both channels in one pass
  • keeps rotation resume correct across the exact startup cap → purge → writeHeader sequence while old files are removed
• full workspace suite — 82 files, 812 passed / 4 skipped
• tsc typecheck (all five tsconfigs) — clean
• root gates (bilingual-docs, architecture, verify-layout) — green

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