[codex] Remove font layer registry

[kostyafarber]

Summary

Removes the obsolete font-level Layer registry from shift-font now that authored glyph data is modeled as Source + GlyphLayer.

This PR:

• removes FontData.layers, default_layer_id, and the associated Font APIs
• deletes the old shift_font::Layer module/export
• updates backend readers and tests to treat imported UFO/designspace/Glyphs layers as sources
• simplifies FontView and bridge export snapshots so they no longer expose font-level layers
• updates bridge JS tests to use sourceId for GlyphLayerRef

Why

The old font-level layer registry duplicated information now carried by sources and glyph layers. Keeping it around made the next workspace semantic API work harder because edits could accidentally target old layer concepts instead of stable source/glyph-layer identity.

Validation

Local commit hooks passed:

• cargo check including tests
• cargo fmt
• cargo clippy
• cargo test
• vitest

Additional manual validation run before commit:

• cargo check --workspace
• cargo test --workspace
• pnpm --filter @shift/desktop typecheck
• pnpm test