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-core/src/client.rs

@@ -172,6 +172,7 @@ impl NativeClient {
         compositor: CompositorPref,
         gamepad: GamepadPref,
         bitrate_kbps: u32,
+        launch: Option<String>,
         pin: Option<[u8; 32]>,
         identity: Option<(String, String)>,
         timeout: Duration,
@@ -214,6 +215,7 @@ impl NativeClient {
                     compositor,
                     gamepad,
                     bitrate_kbps,
+                    launch,
                     pin,
                     identity,
                     frame_tx,
@@ -526,6 +528,7 @@ struct WorkerArgs {
     compositor: CompositorPref,
     gamepad: GamepadPref,
     bitrate_kbps: u32,
+    launch: Option<String>,
     pin: Option<[u8; 32]>,
     identity: Option<(String, String)>,
     frame_tx: SyncSender<Frame>,
@@ -552,6 +555,7 @@ async fn worker_main(args: WorkerArgs) {
         compositor,
         gamepad,
         bitrate_kbps,
+        launch,
         pin,
         identity,
         frame_tx,
@@ -608,6 +612,8 @@ async fn worker_main(args: WorkerArgs) {
                 // No device name yet: the connect ABI has no name parameter (pairing does). The
                 // host falls back to a fingerprint-derived label in its pending-approval list.
                 name: None,
+                // Library id to launch this session, if the embedder asked for one.
+                launch: launch.clone(),
             }
             .encode(),
         )