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/kde/host.env

@@ -1,4 +1,11 @@
 # punktfunk host config for a Fedora/Ubuntu KDE Plasma appliance (kwin backend).
+#
+# APPLIANCE-ONLY: this file deliberately PINS the backend (PUNKTFUNK_COMPOSITOR) and the session
+# env (WAYLAND_DISPLAY/XDG_CURRENT_DESKTOP) at the dedicated headless KWin session — which also
+# turns OFF the host's live-session auto-detection and Desktop<->Game following. On a normal
+# desktop (or any box that switches to Steam Game Mode) do NOT use this file; start from
+# host.env.example instead, whose defaults auto-detect and follow the live session.
+#
 # Copy to ~/.config/punktfunk/host.env. Pairs with punktfunk-kde-session.service, which brings
 # up a headless `kwin --virtual` on wayland-kde (with KWIN_WAYLAND_NO_PERMISSION_CHECKS=1 so the
 # host can bind KWin's privileged zkde_screencast protocol — an interactive Plasma session will