[codex] Add stable ID font index

[kostyafarber]

Summary

Adds the stable-ID font substrate for shift-font.

• Stores glyphs by stable GlyphId in an ordered IndexMap instead of keying authored state by glyph name.
• Adds a private runtime FontIndex beside serializable FontData inside FontState.
• Exposes explicit lookup and identity mutation APIs on Font, including name, Unicode, layer, and glyph/source lookup paths.
• Removes broad attached-glyph mutation from callers and updates workspace, bridge, store, and backend paths to go through Font operations.
• Keeps Font cheap to clone with Arc copy-on-write semantics.

Design Notes

FontData is now authored serializable data only. FontIndex is derived runtime state and is rebuilt/validated when deserializing. Bulk glyph insertion uses incremental index validation/update to avoid quadratic font construction, while broader identity edits still rebuild/validate indexes for now.

Tests

• cargo test --workspace
• pnpm --filter @shift/desktop typecheck
• commit hook also ran cargo check, cargo fmt, cargo clippy, and cargo test

Added coverage for glyph name uniqueness, stable glyph order, Unicode multi-match lookup, layer lookup freshness, serde index rebuilds, and perf marks for rename/Unicode/layer index rebuilds.