Added a Phase 14 Update section to the result_migration_20260616
umbrella spec.md documenting:
- The 2 fixes (Issue 1: GUI subprocess crash; Issue 2: xdist race)
- The final test pass count: 11/11 tiers PASS clean
- Sub-track 2 is now fully ready for merge with no documented issues
- Sub-track 3 (result_migration_app_controller) is unblocked
The Phase 14 update is positioned between section 7 (Commits) and
section 8 (See Also), preserving the existing section numbering.