docs: add Phase 4 (Native Analysis Acceleration) to roadmap

Add new Phase 4 covering the port of JS-only build phases to Rust:
- 4.1-4.3: AST nodes, CFG, dataflow visitor ports (~587ms savings)
- 4.4: Batch SQLite inserts (~143ms)
- 4.5: Role classification & structure (~42ms)
- 4.6: Complete complexity pre-computation
- 4.7: Fix incremental rebuild data loss on native engine
- 4.8: Incremental rebuild performance (target sub-100ms)

Bump old Phases 4-10 to 5-11 with all cross-references updated.
Benchmark evidence shows ~50% of native build time is spent in
JS visitors that run identically on both engines.

Author: carlos-alm