Phase 8: PdfiumPool and Multi-Threading

[tlkahn]

Goal

Provide genuinely safe multi-threaded rendering via a pool of Pdfium instances. Verify all types are !Send.

Tasks

• Verify all types are !Send and !Sync (no accidental auto-trait impls)
• Implement PdfiumSource enum (Path, System, Static, Bundled)
• Implement PdfiumPool: create N Pdfium instances, Mutex-wrapped, semaphore-gated with() method
• Write stress tests: parallel rendering of different PDFs via rayon
• Write examples/parallel_render.rs
• Benchmark: pool rendering vs. sequential rendering throughput

Exit Criterion

Parallel rendering works correctly under rayon with no data races (run under Miri and/or ThreadSanitizer).

Dependencies

• Phase 3: Rendering (MVP) — publish 0.1 #4 (Phase 3 / MVP)
• Independent of Phases 4-7