docs(perf): document Linux refresh_rate query-only behavior (Fluid F10)

[ZhuchkaTriplesix]

Parent

Part of #356 · Fluid perf audit F10 (P2)

Problem

On Linux, refresh_rate enable / prefer-max are no-ops; the plugin only queries GDK monitor refresh. Actual Hz depends on Wayland/X11 compositor. Easy to misread overlay / docs as “unlocked 120 Hz”.

Where:

• lib/core/motion/display_refresh_service.dart
• third_party/refresh_rate Linux plugin
• docs/motion-and-high-refresh.md, docs/perf-baseline.md

Expected

• Explicit “Linux: query-only” callout
• Verification steps for compositor Hz vs overlay
• Table rows that claim Linux 120/144 Hz clarify compositor dependence

Acceptance

• Docs updated (no code unlock required — none available)
• Overlay flag docs mention query vs unlock platforms

Related

• docs/perf-baseline.md §6 Hz verification

[ZhuchkaTriplesix]

Fixed and merged on dev via Fluid perf PRs.