Adds a 255-line fast-path regression harness (`self-healing-fn-5488-fast-path-regressions.test.ts`) covering unbacked merge fast-paths, retry-exhausted fanout, grace-window boundaries, and overlap preservation invariants. Also includes a small cleanup to `MobileNavBar.test.tsx`.
Fusion-Task-Id: FN-5498
Fusion-Task-Lineage: f39ca8a1-223b-4b43-a844-6cb17d19e099
Fusion-Task-Id: FN-5498