feat(host/gamescope): custom-resolution Game-Mode streaming on the Steam Deck

The Steam Deck (SteamOS) ships its OWN gaming session — `gamescope-session.target`
driven by `/usr/lib/steamos/gamescope-session`, not Bazzite's `gamescope-session-plus`.
That script `exec gamescope`s with HARDCODED physical-panel args (`-w 1280 -h 800 -O
'*',eDP-1`) and launches Steam via a SEPARATE `steam-launcher.service`, so the existing
managed-session path (which assumes session-plus) couldn't honor the client's mode — an
attach captured the panel's native 1280x800 instead.

Add a SteamOS branch to the managed-session path: detect it, write a `gamescope` PATH-shim
that rewrites the hardcoded args to `--backend headless -W <client> -H <client> -r <hz>`,
drop a transient user `gamescope-session.service.d` override pointing PATH at the shim +
the mode, then RESTART the whole target so `steam-launcher.service` brings Steam up IN the
headless gamescope at the client's resolution. Attach to the one fresh node (the restart
kills any prior gamescope, so no stale-node attach). Restore-on-disconnect removes the
override + restarts the target back to the physical panel (debounced; skipped if the user
switched to a desktop session). All user-level (`systemctl --user`) — no root.

Also widen `build_pipeline_with_retry` to 8 attempts (~90s): a host-managed gamescope
session cold-starting Steam Big Picture takes 30-60s to first frame, and a first-connect
timeout would tear down the warm session (forcing another cold start on reconnect).
Permanent failures still fail fast via `is_permanent_build_error`.

Validated live on a Steam Deck: Game Mode auto-detected, host takes over headless at the
client's mode (720p / 1080p), Steam Big Picture streamed glass-to-glass to the Mac at the
requested resolution. Single-tenant (concurrent clients at different modes still thrash —
a follow-up).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/punktfunk-host/src/punktfunk1.rs

@@ -2794,7 +2794,12 @@ fn build_pipeline_with_retry(
     bitrate_kbps: u32,
     bit_depth: u8,
 ) -> Result<Pipeline> {
-    const MAX_ATTEMPTS: u32 = 4;
+    // ~10s first-frame wait per attempt. 8 gives a ~90s budget for the SLOW case: a host-managed
+    // gamescope session cold-starting Steam Big Picture (the SteamOS/Bazzite takeover) can take
+    // 30-60s to produce its first frame, and a first-connect timeout would tear down the warm
+    // session (forcing another cold start on reconnect). A genuinely permanent failure still fails
+    // fast via `is_permanent_build_error`; only transient "no frame yet" retries consume the budget.
+    const MAX_ATTEMPTS: u32 = 8;
     let mut backoff = std::time::Duration::from_millis(500);
     for attempt in 1..=MAX_ATTEMPTS {
         match build_pipeline(vd, mode, bitrate_kbps, bit_depth) {