fontique: add persistent cache for system font scans on macOS

[nicoburns]

Summary

Stacked on #3. Adds a persistent scan cache so warm app startups skip parsing font files entirely.

New scan_cache module stores, per scanned file, a FileStamp { mtime_secs, mtime_nanos, size } plus the extracted FontRecords (family names, postscript name, index, width/style/weight, variation axes, CharmapIndex). ScannedCollection::from_paths_cached(paths, max_depth, cache_path) stats each discovered file, reuses cached records when the stamp matches, parses only new/changed files in parallel, and rewrites the cache when anything was parsed or a cached file disappeared:

cache hit  = stamp(path) == cached.stamp  -> reuse records (fresh SourceId per run)
cache miss -> parse in parallel           -> records added to rewritten cache

Format is a hand-rolled little-endian binary encoding (no new dependencies) with a magic number + version; any decode failure falls back to a full rescan that rewrites the cache. Writes go to a temp file + rename so readers never see a partial cache. Runtime IDs (SourceId/FamilyId) are never persisted — they're regenerated on load. attr_axes is recomputed from the cached axes rather than stored.

The CoreText backend now uses ~/Library/Caches/fontique/font-scan-cache.bin (falling back to an uncached scan if home_dir() is unavailable). Open question for review: whether the cache path should instead be configurable via API (e.g. on CollectionOptions, which is currently Copy) — happy to change the plumbing.

Includes a hermetic test (using parley_dev font assets) verifying every cached attribute round-trips identically and that changed files are re-parsed.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/60a3d833a0894a73945c806adff1cbf3
Requested by: @nicoburns

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