fix(host/security): close audit findings S1,#1,#4,#10,#12,#7,#6,S2-S6 (Linux/cross-platform)

Remediations from design/security-review-2026-06-28.md verified on Linux
(cargo check/clippy/test green; Windows-gated paths verify in CI):

- S1 [HIGH]: bump quinn-proto 0.11.14 -> 0.11.15 (RUSTSEC-2026-0185,
  pre-auth out-of-order STREAM reassembly memory exhaustion on the
  always-on default QUIC listener).
- #1 [HIGH]: remove the unauthenticated nvhttp `GET /pin` endpoint; the
  GameStream PIN is delivered ONLY via the bearer-gated mgmt API, so a
  network client can no longer submit its own displayed PIN and self-pair.
- #4 [HIGH->MED]: gate the unauthenticated RTSP/UDP media plane on a paired
  `/launch` and bind it to the launching client's source IP (threaded
  through the HTTPS handler), so an unpaired peer can neither start capture
  on an idle host nor ride a paired client's active launch.
- #12: bound concurrent parked pairing waiters (MAX_PARKED_WAITERS) so a
  pre-auth peer can't pin unbounded 300s handshakes. +regression test.
- #10: throttle the per-packet ENet control GCM-decrypt-failed warn
  (exponential backoff) so a junk flood can't spam the log.
- #7 [MED->LOW]: serialize all process-global env mutation on the
  session-setup path under a new vdisplay::ENV_LOCK (apply_session_env /
  apply_input_env / the launch-cmd set_var / the gamescope env read), so
  concurrent native sessions can't race set_var/getenv (data-race UB ->
  host-wide DoS). Full per-session SessionContext threading remains a
  follow-up for cross-session value confusion.
- #6 [MED]: move the gamescope EIS socket relay from world-writable /tmp to
  $XDG_RUNTIME_DIR (per-user 0700) and reject a symlinked relay file, so a
  local user can't intercept (keylog) or deny the remote session's input.
- S2: a malformed client Opus mic frame now drops that frame instead of
  tearing down the shared host-lifetime virtual mic (cross-session DoS).
- S3: track held buttons/keys in capped HashSets (was unbounded Vec with
  O(n) scans) so a paired client can't grow per-session input state.
- S5: reject fps==0/absurd at the open_video chokepoint (covers Hello,
  ANNOUNCE, Reconfigure) so the encoder time_base/pts math can't div-by-0.
- S6: bound the shared mic mpsc (drop-newest when full).
- S4: cap Epic launcher-cache reads (catcache.bin/.item) so a planted giant
  can't OOM the host during library enumeration.

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

crates/punktfunk-host/src/gamestream/mod.rs

@@ -90,6 +90,11 @@ pub struct LaunchSession {
     pub fps: u32,
     /// `/launch?appid=N` — selects the app-catalog entry (session recipe).
     pub appid: u32,
+    /// Source IP of the paired HTTPS client that issued `/launch`. The unauthenticated RTSP/UDP
+    /// media plane binds to this so only the launching peer can start/own the stream — an
+    /// unpaired RTSP peer cannot ride a paired client's launch (security-review 2026-06-28 #4).
+    /// `None` if the address could not be captured (then RTSP falls back to launch-present only).
+    pub peer_ip: Option<std::net::IpAddr>,
 }
 
 /// Shared control-plane state used as the axum app state.