docs(env): stop teaching the compositor pin + uid-1000 anchors in starters

Field triage (Nobara, Discord): the kde.md starter host.env told desktop
users to set PUNKTFUNK_COMPOSITOR=kwin, which PINS the backend — detect()
short-circuits and the capture-loss rebuild never re-detects — so a
mid-stream switch to Game Mode killed the stream instead of following it.
A follow-up hardcoded XDG_RUNTIME_DIR=/run/user/1000 anchor broke PipeWire
for any non-1000 uid (pw audio connect: Creation failed).

Revamp across every starter/example/reference:
- Desktop starters (kde/gnome/hyprland/sway) shrink to
  PUNKTFUNK_VIDEO_SOURCE=virtual + an explicit warning that pinning
  disables session-following; forcing a backend is CI/appliance-only.
- host.env.example: rewritten around auto-detection; anchors demoted to a
  commented ssh/cron-only block with the uid trap spelled out; the
  gamescope ATTACH/MANAGED knobs documented (previously missing);
  case-sensitivity called out.
- packaging/bazzite/host.env + README: drop the uid-1000 anchors (a
  systemctl --user service inherits/derives them); README's stale
  PUNKTFUNK_COMPOSITOR=gamescope-era template synced to the real one.
- packaging/kde/host.env: loud APPLIANCE-ONLY header (it pins on purpose).
- configuration.md: session-anchors section inverted to "leave unset",
  compositor row states the pin consequence, case-sensitivity note.
- troubleshooting.md: new "session fails right after editing host.env"
  section (case, wrong-uid anchors, stale pin, restart-to-apply).
- gamescope.md/bazzite.md: attach/managed descriptions match current
  behavior (managed is the infra-detected default; attach re-modes a
  box-owned session to the client's resolution).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packaging/nix/README.md

@@ -168,14 +168,16 @@ Everything the RPM's `%install` + `%post` do, declaratively:
 
 ### Headless / appliance
 
-Set `autoStart = true`, enable lingering, and pick a backend in `settings`:
+Set `autoStart = true`, enable lingering, and — for a **dedicated single-session appliance** —
+pin a backend in `settings` (pinning `PUNKTFUNK_COMPOSITOR` disables live-session auto-detection,
+so leave it out on any box that switches between a desktop and Game Mode):
 
 ```nix
 services.punktfunk.host = {
   enable = true;
   autoStart = true;
   users = [ "streamer" ];
-  settings = { PUNKTFUNK_COMPOSITOR = "gamescope"; };  # or kwin/mutter/wlroots
+  settings = { PUNKTFUNK_COMPOSITOR = "gamescope"; };  # appliance-only; omit to auto-detect
 };
 users.users.streamer.linger = true;
 # For the gamescope/KWin backends extend the service PATH, e.g.: