perf(ui): narrow ResultsTab mode morph paint scope (Fluid F8)

[ZhuchkaTriplesix]

Parent

Part of #356 · Fluid perf audit F8 (P1)

Problem

Results mode morph (QueryaFadeSlide) applies opacity over toolbar + virtualized grid on idle → loading → grid/error. Grid itself correctly avoids per-row scroll animation, but whole-grid opacity is still expensive.

Where: lib/features/main_screen/results_tab.dart, result_grid_view.dart

Expected

• RepaintBoundary around morph child (or grid)
• Prefer morphing chrome / lightweight states; hard-swap heavy grid when reasonable
• Keep mode keys (not per-row keys)

Acceptance

• Mode morph does not unnecessarily opacity-composite the full virtual grid when avoidable
• Scroll still has no row fade
• Existing results_tab tests pass / extended

Related

• Checklist: docs/perf-baseline.md §12 Results modes

[ZhuchkaTriplesix]

Fixed and merged on dev via Fluid perf PRs.