fontique: parallelize file system font scanning with sparse reads

[nicoburns]

Summary

Speeds up ScannedCollection::from_paths (used for system font enumeration on macOS/Android and load_fonts_from_paths) by restructuring the scan into three phases:

1. collect_font_files: walk the given paths up front into a deduplicated file list.
2. parse_files: parse files in parallel across available_parallelism() threads (std::thread::scope + an atomic cursor for work stealing), each producing plain FontRecord { names, postscript_name, font } values.
3. Merge records into the FamilyNameMap/FamilyInfo/postscript-name maps on the calling thread (these maps allocate runtime IDs and aren't thread-safe).

Per-file parsing also switches from mmap to positioned read calls (mmap page faults serialize in the kernel, defeating parallelism). Rather than reading whole files, read_font_metadata reads only what scanning needs — header, TTC directory, table directories, and the name/OS/2/head/post/fvar/cmap tables — merging overlapping ranges and placing them at their original offsets in a sparse buffer so FontRef::from_index sees a normal font. This matters for large fonts (CJK, emoji, .ttc collections) where metadata is a tiny fraction of the file.

Behavior is preserved: same family/alias/postscript-name results, hvgl fonts still rejected, invalid fonts skipped. Verified on Linux that the sparse-read parse produces byte-identical results (indices, names, postscript names, charmap offsets) to the previous mmap scan across all 236 system font files including a .ttc.

Note: I can only cross-compile for macOS from this environment (clippy/check pass for aarch64-apple-darwin, Windows, wasm, Linux, and a no-std target); real-world timing on macOS still needs to be measured. Stacked PR adding a persistent scan cache follows.

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

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