feat(vdisplay/mutter): pin the client's refresh by default; drop PUNKTFUNK_MUTTER_VIRTUAL_REFRESH

The >60 Hz virtual-monitor path (RecordVirtual "modes" with the client's exact WxH@Hz)
was gated behind PUNKTFUNK_MUTTER_VIRTUAL_REFRESH, default OFF, after a high-refresh
virtual CRTC SIGSEGV'd gnome-shell on session teardown. That crash was since fixed by
stopping the screencast before any monitor reconfig, so the gate is dead weight — and a
silent footgun: every non-headless GNOME client was capped at Mutter's PipeWire-derived
60 Hz unless they knew the hidden flag.

Make it the default: the custom-mode path now runs whenever mode.refresh_hz > 60 (≤60 Hz
stays byte-identical to before — Mutter's 60 Hz default is already correct), and the
virtual_refresh_enabled() env read is removed. Docs updated (configuration.md env table,
vrr-plan.md reference).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs-site/content/docs/configuration.md

@@ -72,7 +72,6 @@ picture.
 |---|---|---|
 | `PUNKTFUNK_KWIN_VIRTUAL_PRIMARY` | `1` | Make the streamed per-session output the sole desktop so plasmashell + windows render on it (not on the headless bootstrap output). Set by the KDE appliance `host.env`. Superseded by the console's **Topology** setting. |
 | `PUNKTFUNK_MUTTER_VIRTUAL_PRIMARY` | `1` | GNOME/Mutter equivalent of the above. |
-| `PUNKTFUNK_MUTTER_VIRTUAL_REFRESH` | `1` | Pin the client's exact WxH**@Hz** via `RecordVirtual`'s custom modes (needed for >60 Hz on Mutter). |
 
 ## Video quality