feat(launch): punktfunk/1 launch integration — client picks a title, host runs it

Plan step 4 (plumbing + host behavior). A client can ask the host to launch a
library title on connect; the host resolves it against ITS OWN library and runs it
in the session — the client sends only the store-qualified id, never a command, so a
remote peer can't inject one.

- Protocol (quic.rs): `Hello.launch: Option<String>` (the GameEntry id). Appended
  after `name`; when launch is present but name absent, a zero-length name placeholder
  keeps the offset deterministic — so a Hello with neither field stays byte-identical
  to the bitrate-era 26-byte form (test-asserted). Old peers ignore it; new hosts
  decode None from old clients. Round-trip + back-compat + truncation tests.
- Host: `library::launch_command(id)` resolves id → command via the host's own library —
  `steam_appid` → `steam steam://rungameid/<appid>` (appid validated as digits, the only
  client-influenced part), `command` → the host-stored command verbatim (trusted, never
  from the client). m3.rs sets PUNKTFUNK_GAMESCOPE_APP from it before bringup, exactly
  as the GameStream /launch path does (one session at a time). Unit-tested incl. an
  injection-attempt guard. Takes effect on the bare-spawn gamescope path; a no-op on a
  shared desktop / attach-to-existing session.
- C ABI: `punktfunk_connect_ex4` adds `launch_id` (NULL = none); `_ex3` now delegates to
  it. Threaded through NativeClient::connect → WorkerArgs → Hello.
- client-rs gains `--launch ID` (headless testing); client-linux passes None (no picker
  yet). Header regenerated.

Next: the Apple library grid passes the picked id via punktfunk_connect_ex4.

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

crates/punktfunk-host/src/m3.rs

@@ -520,6 +520,24 @@ async fn serve_session(
             M3Source::Synthetic => None,
         };
 
+        // Resolve a requested library launch (the client sends only the store-qualified id;
+        // we look it up in OUR library so a client can't inject a command). Set the gamescope
+        // backend's app env var, exactly as the GameStream /launch path does — safe per-session
+        // (one session at a time). Only the bare-spawn gamescope path reads it; on a shared
+        // desktop (kwin/mutter/wlroots) or an attach-to-existing session it's a harmless no-op.
+        if let Some(id) = hello.launch.as_deref() {
+            match crate::library::launch_command(id) {
+                Some(cmd) => {
+                    tracing::info!(launch_id = id, command = %cmd, "launching library title");
+                    std::env::set_var("PUNKTFUNK_GAMESCOPE_APP", &cmd);
+                }
+                None => tracing::warn!(
+                    launch_id = id,
+                    "client requested a launch id not in this host's library — ignoring"
+                ),
+            }
+        }
+
         // Resolve the client's gamepad-backend preference (pure env/cfg check — no probing
         // needed; the actual pads are created lazily by the input thread).
         let gamepad = resolve_gamepad(hello.gamepad);
@@ -2389,6 +2407,7 @@ mod tests {
                 CompositorPref::Auto,
                 GamepadPref::Auto,
                 0,
+                None, // launch
                 None,
                 Some((cert.clone(), key.clone())),
                 timeout
@@ -2419,6 +2438,7 @@ mod tests {
             CompositorPref::Auto,
             GamepadPref::Auto,
             0,
+            None, // launch
             None,
             Some((cert, key)),
             timeout,
@@ -2478,6 +2498,7 @@ mod tests {
                 CompositorPref::Auto,
                 GamepadPref::Auto,
                 0,
+                None, // launch
                 None,
                 None,
                 timeout
@@ -2503,6 +2524,7 @@ mod tests {
             CompositorPref::Auto,
             GamepadPref::Auto,
             0,
+            None, // launch
             Some(host_fp),
             Some((cert.clone(), key.clone())),
             timeout,