Add docs/PERFORMANCE.md documenting the post-Phase-4 startup slowdown:
the ~290ms increase is entirely Fyne 2.5.3->2.6.3's w.Show() cost, not
the Service/domain/UI refactor. Includes the GOSENTRY_TIMING method and
warm-run results table so it can be reproduced.
Wire the startup re-measurement into the existing Fyne 2.6.3->2.7.x
upgrade task in ROADMAP.md so 2.7's effect on w.Show() gets checked.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
mixeme