fontique: enumerate macOS system fonts via CTFontManagerCopyAvailableFontURLs

[nicoburns]

Summary

Speeds up system font enumeration in the CoreText backend on macOS. scan_system_fonts() previously did CTFontCollection::from_available_fonts() → matching_font_descriptors() → per-descriptor kCTFontURLAttribute lookup, which materializes a font descriptor for every face just to recover file URLs. On macOS this is replaced with CTFontManagerCopyAvailableFontURLs() (already exposed by objc2-core-text behind the CTFontManager feature), which returns the font file URLs directly.

The enumeration is factored into system_font_files() -> Option<HashSet<PathBuf>>:

• #[cfg(target_os = "macos")]: CTFontManagerCopyAvailableFontURLs → POSIX paths → dedupe.
• #[cfg(not(target_os = "macos"))] (iOS etc.): the previous descriptor-based path, unchanged — CTFontManagerCopyAvailableFontURLs is macOS-only and referencing it on other Apple targets would fail to link.

Behavior is otherwise unchanged: paths are deduped into a HashSet<PathBuf>, still extended with library_font_files() (Apple hides SFNS.ttf etc. from these APIs), and scan_system_fonts() still returns None on failure/empty so the caller falls back.

Deliberately narrow: parallel scanning and caching are being done separately.

Verified with cargo check/clippy -p fontique --features system for aarch64-apple-darwin and aarch64-apple-ios, plus cargo fmt --check.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/5893bf8aee24455699895691e86d1c32
Requested by: @nicoburns

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