fix(vdisplay/gamescope): a fresh dedicated launch streams from the first second

A freshly spawned headless gamescope never delivered a single PipeWire
buffer when a game was launched: gamescope only composites (and only on a
composite pushes a capture buffer) when a client paints, and a nested
Steam bootstrap paints nothing until its UI's first frame — far longer
than the 10 s first-frame budget. The native plane's retry ladder then
killed the half-booted Steam on every attempt and started from zero; the
GameStream plane died on its single wait. Reused live displays worked,
which pointed away from the real cause (root-caused on .41 with a raw
pw_stream probe: `sleep infinity` nested → 0 buffers ever, `vkcube`
nested → 60/s immediately).

Every bare spawn now backgrounds the host's own splash client
(`gamescope-splash`, hidden subcommand) beside the nested app: a dark
window with a subtle breathing bar painted at ~2.5 Hz — damage from the
first second, so capture gets its first frame within the budget on both
planes. In `--steam` mode gamescope composites only windows whose appid
is in the root `GAMESCOPECTRL_BASELAYER_APPID` list (live-proven: even a
painting vkcube gets zero composites without it), so the splash declares
itself as the Steam UI (STEAM_GAME=769) and seeds the baselayer iff
unset; Steam's own rewrite at game launch hands composite focus to the
game with no action on our side. PUNKTFUNK_GAMESCOPE_SPLASH=0 is the
escape hatch.

The dedicated Steam launch is also shaped with `-gamepadui` instead of
`-silent`: the nested Steam is Big Picture — the identity gamescope's
`--steam` integration is built around — so the boot shows the gamepad UI
instead of the desktop client window flashing through the stream, and
gamescope's focus rules (game outranks the Steam UI appid) cover what
`-silent` was working around.

On-glass on .41 (Bazzite, RTX 5070 Ti, gamescope c31743d): cold connect +
Balatro launch delivers frames on the first attempt on both a cold and a
torn-down-then-fresh spawn; Big Picture boot → game handover confirmed.

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

crates/punktfunk-host/src/main.rs

@@ -333,6 +333,11 @@ fn real_main() -> Result<()> {
         // names the inherited socketpair end.
         #[cfg(target_os = "linux")]
         Some("zerocopy-worker") => zerocopy::worker::run_from_args(&args[1..]),
+        // Hidden: the splash client every bare gamescope spawn backgrounds beside its nested app,
+        // so a fresh headless gamescope composites (and its PipeWire node delivers frames) from
+        // the first second — never run by hand; it needs a gamescope session's DISPLAY.
+        #[cfg(target_os = "linux")]
+        Some("gamescope-splash") => vdisplay::gamescope_splash_client(),
         // NV12 colour self-test (no display/capture needed): convert a known RGBA pattern to NV12
         // on the GPU and compare against a BT.709 limited-range reference. Validates the Tier 2A
         // `PUNKTFUNK_NV12` convert is colour-correct. Prints PASS/FAIL + max Y/U/V error.