docs: mark Phase 2 DONE, add integration status and progress tracker Phase 2 (bgz17 container/semiring) verified complete — 121 tests passing. Added cross-repo integration status section to FINAL_STACK.md. Created PROGRESS.md tracking Plateaus 0-3 against master integration plan. https://claude.ai/code/session_01CdqyUTUfjKZuk8YGJzv6LB#35