feat(host/linux): PUNKTFUNK_GAMESCOPE_STEAM opt-in for bare gamescope spawns

Adds --steam (before the -- terminator, where PUNKTFUNK_GAMESCOPE_APP
cannot reach) to the bare headless gamescope spawn when the env var is
truthy, enabling gamescope's Steam integration for steam -gamepadui
dedicated sessions. Default off; managed gamescope-session-plus/SteamOS
sessions own their own flags and are unaffected.

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

crates/punktfunk-host/src/config.rs

@@ -71,6 +71,10 @@ pub struct HostConfig {
     /// backend (the legacy SudoVDA backend was removed), so this is currently informational — kept for the
     /// shipped `host.env` and as a forward seam if a second backend is ever added.
     pub vdisplay: Option<String>,
+    /// `PUNKTFUNK_GAMESCOPE_STEAM` — opt the bare headless gamescope spawn into its Steam
+    /// integration mode (`--steam`). Managed gamescope-session-plus/SteamOS sessions own their
+    /// own flags and do not consult this.
+    pub gamescope_steam: bool,
     /// `PUNKTFUNK_RECOVER_SESSION_CMD` — operator hook fired (debounced) when a client connects while NO
     /// graphical session is live for this uid: the state a compositor crash leaves behind (gnome-shell
     /// SIGSEGV → GDM greeter, whose auto-login is once-per-boot, so the box would otherwise need a walk-up
@@ -120,6 +124,12 @@ impl HostConfig {
             compositor: val("PUNKTFUNK_COMPOSITOR"),
             gamepad: val("PUNKTFUNK_GAMEPAD"),
             vdisplay: val("PUNKTFUNK_VDISPLAY"),
+            gamescope_steam: val("PUNKTFUNK_GAMESCOPE_STEAM").is_some_and(|s| {
+                matches!(
+                    s.trim().to_ascii_lowercase().as_str(),
+                    "1" | "true" | "yes" | "on"
+                )
+            }),
             recover_session_cmd: val("PUNKTFUNK_RECOVER_SESSION_CMD")
                 .filter(|s| !s.trim().is_empty()),
         }

crates/punktfunk-host/src/vdisplay/linux/gamescope.rs

@@ -1359,6 +1359,21 @@ fn shape_dedicated_command(app: &str) -> String {
     app.to_string()
 }
 
+/// Add the compositor-side arguments shared by every bare gamescope spawn. `steam_mode` belongs
+/// before the `--` terminator; [`PUNKTFUNK_GAMESCOPE_APP`](spawn) configures the nested command
+/// after it and therefore cannot enable gamescope's Steam integration itself.
+fn add_bare_gamescope_args(command: &mut Command, w: u32, h: u32, hz: u32, steam_mode: bool) {
+    command
+        .args(["--backend", "headless"])
+        .args(["-W", &w.to_string()])
+        .args(["-H", &h.to_string()])
+        .args(["-r", &hz.to_string()]);
+    if steam_mode {
+        command.arg("--steam");
+    }
+    command.args(["--xwayland-count", "1", "--"]);
+}
+
 /// Spawn `gamescope --backend headless -W w -H h -r hz -- <app>`. The app comes from
 /// `PUNKTFUNK_GAMESCOPE_APP` (default a no-op that just keeps gamescope alive — set it to a real
 /// game/GL app for actual content, e.g. `steam -gamepadui` for the SteamOS-like session).
@@ -1385,13 +1400,10 @@ fn spawn(w: u32, h: u32, hz: u32, cmd: Option<&str>, log: &std::path::Path) -> R
     let app = shape_dedicated_command(&app);
     let relay = ei_socket_file();
     let _ = std::fs::remove_file(&relay); // stale socket path from a previous session
+    let steam_mode = crate::config::config().gamescope_steam;
     let mut cmd = Command::new("gamescope");
-    cmd.args(["--backend", "headless"])
+    add_bare_gamescope_args(&mut cmd, w, h, hz, steam_mode);
-        .args(["-W", &w.to_string()])
+    cmd.args([
-        .args(["-H", &h.to_string()])
-        .args(["-r", &hz.to_string()])
-        .args(["--xwayland-count", "1", "--"])
-        .args([
         "sh",
         "-c",
         &format!(
@@ -1419,7 +1431,7 @@ fn spawn(w: u32, h: u32, hz: u32, cmd: Option<&str>, log: &std::path::Path) -> R
     } else {
         cmd.stdout(Stdio::null()).stderr(Stdio::null());
     }
-    tracing::info!(w, h, hz, %app, log = %log.display(), "spawning gamescope (headless)");
+    tracing::info!(w, h, hz, steam_mode, %app, log = %log.display(), "spawning gamescope (headless)");
     cmd.spawn()
         .context("spawn gamescope (is it installed? `apt install gamescope`)")
 }