Phase 4 Step 4: @font-face / custom web fonts

[thomasnemer]

Parent: #41

Goal

Implement @font-face support and custom web font loading so that pages can use fonts fetched from the network.

Prerequisites

• Steps 2-3 (render pipeline stable before adding font complexity)

Implementation

• Parse @font-face rule: font-family, src (url + format hints), font-weight, font-style, font-display, unicode-range
• Font loader: fetch WOFF2/WOFF/TTF via ie-net
• WOFF2 decompression using woff2 crate
• Disk + memory font cache (avoid re-downloading)
• Register loaded fonts into fontdb/cosmic-text so glyphon picks them up
• Implement font-display behavior state machine per @font-face: swap, block, fallback, optional
• Trigger relayout when a font transitions from loading to loaded state

Tests

• @font-face parsing tests for all descriptors
• WOFF2 decompression round-trip tests
• Font cache hit/miss tests
• font-display state machine transition tests
• Integration test: page with custom font loads, relayouts, renders correctly

Acceptance Criteria

• @font-face rules parse correctly in ie-css
• WOFF2, WOFF, and TTF fonts load and decompress correctly
• Fonts are cached on disk and in memory
• font-display behaviors match spec (swap, block, fallback, optional)
• Layout updates when custom fonts finish loading