perf(ui): keep theme cross-fade gated; document rebuild cost (Fluid F5)

[ZhuchkaTriplesix]

Parent

Part of #356 · Fluid perf audit F5 (P1)

Problem

AnimatedQueryaTheme + ShadcnAnimatedTheme push InheritedTheme updates every tick → app-wide rebuilds during theme morph. Already gated by preference + motion level, but cost is easy to regress if defaults change.

Where: lib/app/app.dart, theme motion helpers, animated_querya_theme.dart

Expected

• Theme animation remains opt-in (not default-on)
• Docs call out rebuild cost @ 120 Hz
• Motion Off / preference off → snap (already)

Acceptance

• Confirm defaults stay safe (no silent enable)
• Short doc note in docs/motion-and-high-refresh.md or docs/perf-baseline.md §14
• No behavior change required unless a default is wrong

Related

• Checklist: docs/perf-baseline.md §14 Theme cross-fade

[ZhuchkaTriplesix]

Fixed and merged on dev via Fluid perf PRs.