docs: PROJECT.md has stale Phase 3 status, wrong MSRV, Title Case headers, and em dash

- L43-44: says "15/16 Phase 3 prompts merged. Only P3-08 remains" but Phase 3 is complete (all 16 merged)
- L49: MSRV listed as 1.85, actual is 1.92 (bumped in PR #108)
- Title Case headers: `Key Decisions` → `Key decisions`, `Current State` → `Current state`, `Common Gotchas` → `Common gotchas`
- L48: em dash in "CI catches cargo fmt consistently — always run before pushing" → use period