Add Type 3 glyph rasterizer for Phase 2.5 shape recognition (Level 4 fallback).
- Add type3_rasterizer.rs module with:
- Bitmap32x32: 32x32 grayscale bitmap (0=black ink, 255=white paper)
- PathCommand enum and CurrentPath for path construction
- RasterizerContext for content stream execution
- Supported operators: m l c v y re h n S s f F f* B B* b b* q Q cm Do
- Stack depth limit: 20 levels
- Simple scanline rasterization for rectangles
- Add raster_cache field to Type3Font:
- DashMap-based thread-safe cache for rasterized bitmaps
- get_cached_bitmap(), cache_bitmap(), raster_cache() methods
- Public API: rasterize_type3_glyph(font, glyph_name) -> Option<[u8; 1024]>
Acceptance criteria:
- PASS: 32x32 square rasterizes to half-filled bitmap
- PASS: Form XObject recursion limited to 20 levels
- PASS: Unknown glyph returns None without panic
- WARN: FontBBox fallback not yet implemented (requires /FontBBox access)
Tests: All 13 type3_rasterizer tests pass (218 total font module tests pass)
Closes: pdftract-15qr
