Add comprehensive Phase 6 documentation and testing artifacts: a high-level PHASES-1-6 summary, a Phase 6 quick-start checklist, and a detailed Phase 6 testing & iteration guide. Also append a Phase 6 entry to docs/dev-log.md describing the testing framework, test cases (including the critical velocity→animation test), success criteria, and next steps for in-game validation. These docs prepare the repo for Phase 6 in-game testing and outline debugging, performance benchmarks, and iteration procedures.
