FN-5708: backfill files changed chip on done task cards

Ensure freshly done task cards show the "N files changed" chip once mergeDetails diff stats arrive.

- backfill TaskCard diff stat display logic so completed cards update when mergeDetails enrichment lands after initial render
- update useTaskDiffStats to derive files-changed counts from merge details with fallback-safe behavior
- add TaskCard and useTaskDiffStats test coverage for late-arriving mergeDetails and chip rendering
- add a changeset for @runfusion/fusion documenting the patch release

Files changed:
 .changeset/fn-5708-files-changed-chip.md           |  5 ++
 packages/dashboard/app/components/TaskCard.tsx     | 10 +++
 .../app/components/__tests__/TaskCard.test.tsx     | 64 +++++++++++++++
 .../app/hooks/__tests__/useTaskDiffStats.test.ts   | 92 ++++++++++++++++++++++
 packages/dashboard/app/hooks/useTaskDiffStats.ts   | 17 +++-
 5 files changed, 185 insertions(+), 3 deletions(-)

Fusion-Task-Id: FN-5708

Fusion-Task-Lineage: 3f961f3f-045b-473e-9636-ca33fd84282c

.changeset/fn-5708-files-changed-chip.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Backfill done-task "N files changed" chips when mergeDetails enrichment arrives after the initial done websocket snapshot. Task cards now pass a done-mode merge enrichment signature into diff-stats invalidation so `/api/tasks/:id/diff` is re-fetched and authoritative lineage stats render without requiring a manual refresh.