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>

packaging/bazzite/host.env

@@ -1,26 +1,25 @@
 # punktfunk host config for Bazzite (~/.config/punktfunk/host.env).
 #
-# Bazzite ships gamescope, PipeWire and the NVIDIA driver, so the default backend here is
-# gamescope: the host spawns a headless gamescope per session at the client's exact mode and
-# captures its PipeWire node — no separate desktop session to bring up. Set PUNKTFUNK_GAMESCOPE_APP
-# to what you want to run inside it (e.g. `steam -gamepadui` for a SteamOS-like couch session).
+# The compositor + input backend are AUTO-DETECTED per connect from the ACTIVE session: the host
+# follows the box as you flip between Steam Gaming Mode (gamescope — attached to the running
+# session, no churn) and a KDE/GNOME Desktop (KWin/Mutter virtual output at the client's mode).
+# So nothing here forces a backend — only the trustworthy anchors stay.
 
 XDG_RUNTIME_DIR=/run/user/1000
 DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
 
-# gamescope backend: spawned per session, no compositor login required.
-PUNKTFUNK_COMPOSITOR=gamescope
 PUNKTFUNK_VIDEO_SOURCE=virtual
-PUNKTFUNK_GAMESCOPE_APP=steam -gamepadui
-
-# gamescope hosts its own EIS input socket — input lands in the nested session.
-PUNKTFUNK_INPUT_BACKEND=gamescope
 
 # GPU zero-copy capture (dmabuf -> CUDA -> NVENC). Auto-falls back to CPU if unavailable.
 PUNKTFUNK_ZEROCOPY=1
 
 #RUST_LOG=info
 
-# To drive the full Plasma/GNOME desktop instead of a nested gamescope, switch to:
-#   PUNKTFUNK_COMPOSITOR=kwin    (and run inside a KDE session — WAYLAND_DISPLAY/XDG_CURRENT_DESKTOP set)
-#   PUNKTFUNK_INPUT_BACKEND=libei
+# --- Optional overrides (default is active-session auto-detection) ---
+# Force a specific backend for testing (skips auto-detect + env retargeting):
+#   PUNKTFUNK_COMPOSITOR=kwin|mutter|wlroots|gamescope
+#   PUNKTFUNK_INPUT_BACKEND=libei|wlr|gamescope|uinput
+# Opt into the host-MANAGED gamescope session (spawns gamescope-session-plus at the client mode,
+# stops the autologin gaming session for the duration) instead of attaching to the running one:
+#   PUNKTFUNK_GAMESCOPE_MANAGED=1
+#   PUNKTFUNK_GAMESCOPE_APP=steam -gamepadui