feat(host/vdisplay): per-connect active-session backend selection

Bazzite/SteamOS boxes flip between Steam Gaming Mode (gamescope) and a
KDE/GNOME desktop. The host statically read PUNKTFUNK_COMPOSITOR /
XDG_CURRENT_DESKTOP once, so switching to Desktop Mode failed the stream, and
the gamescope managed-session path stopped+relaunched the autologin per connect
— leaking GPU context on F44 (reconnect → black screen).

Replace the static read with a runtime probe of the live session and route each
connect to the right backend, churn-free:

- vdisplay::detect_active_session() probes /proc for the running compositor of
  our uid (gamescope|kwin_wayland|gnome-shell|sway, desktop outranks a leftover
  gamescope) + scans the runtime dir for the live wayland-* socket. Returns an
  ActiveKind + the SessionEnv (WAYLAND_DISPLAY/XDG_RUNTIME_DIR/DBUS/
  XDG_CURRENT_DESKTOP) that targets it.
- apply_session_env() writes that into the process env per connect (host serves
  one session at a time), so every backend (capture + input) opens against the
  live session; apply_input_env() points input at the matching backend and
  selects gamescope ATTACH (no managed restart) unless PUNKTFUNK_GAMESCOPE_MANAGED.
- resolve_compositor() (native path) auto-detects + applies; explicit
  PUNKTFUNK_COMPOSITOR still wins (legacy/CI/forcing). detect() is now
  active-aware for the GameStream/mgmt callers too.
- Bazzite host.env drops the static gamescope force; documents auto-detection
  + the optional overrides.

Result: Desktop Mode → KWin/Mutter virtual output at the client's mode
(churn-free, the reliable path); Gaming Mode → attach to the running gamescope
(no SIGSEGV/GPU leak on reconnect). Compiles + clippy-clean; 78 host tests pass.
Live validation on the Bazzite box pending (box offline).

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

scripts/host.env.example

@@ -1,4 +1,10 @@
 # punktfunk host configuration (~/.config/punktfunk/host.env) — consumed by punktfunk-host.service.
+#
+# The compositor + input backend are AUTO-DETECTED per connect from the live session (the host
+# probes which compositor is actually running and retargets WAYLAND_DISPLAY/XDG_CURRENT_DESKTOP/
+# DBUS at it), so a box that flips between Steam Gaming Mode and a KDE/GNOME desktop is followed
+# automatically. The blocks below are OPTIONAL OVERRIDES — uncomment one only to force a backend
+# (this also skips the per-connect env retargeting). The anchors XDG_RUNTIME_DIR + DBUS stay.
 
 # Session / compositor environment (headless KWin example).
 XDG_RUNTIME_DIR=/run/user/1000