feat(punktfunk/1): request-IDR recovery for a wedged client decode

Fixes the intermittent first-connect freeze. The host streams infinite GOP — one
opening IDR, then P-frames only (recovery keyframes just on loss) — so when the
client's decoder wedges on the cold first session (a lost/corrupt opening IDR, a
bad early P-frame) the picture stays frozen until the far-off next keyframe. The
client had no way to ask for one; now it does.

Add a RequestKeyframe control message (client -> host, reliable control stream),
mirroring Reconfigure:
- core: quic.rs RequestKeyframe (type 0x03) + roundtrip test; client.rs
  CtrlRequest::Keyframe + NativeClient::request_keyframe; abi.rs
  punktfunk_connection_request_keyframe (header regenerated).
- host: m3.rs decodes it in the control loop and signals the encode loop, which
  coalesces a burst and calls enc.request_keyframe() — wiring the existing
  NvencEncoder hook (force_kf -> next frame pict_type=I), the same recovery the
  GameStream path already had via force_idr.
- apple: PunktfunkConnection.requestKeyframe(); StreamPump (stage-1) requests on
  layer.status==.failed; Stage2Pipeline (stage-2) on a sync submit failure and on
  the async decode-error callback via a thread-safe KeyframeRecovery. All
  throttled to <=1/250ms (the decode stays wedged for several frames until the IDR
  lands, so per-frame requests would flood the control stream).

Self-healing: a lost recovery IDR is re-requested after the throttle; the host
coalesces bursts into a single IDR.

Validated: cargo fmt + clippy clean; core + host test suites green (incl. new
request_keyframe_roundtrip); swift build + test (39 passed); xcframework rebuilt
(all 5 slices), header regenerated with no unrelated drift.

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

clients/apple/Sources/PunktfunkKit/PunktfunkConnection.swift

@@ -332,6 +332,21 @@ public final class PunktfunkConnection {
         _ = punktfunk_connection_request_mode(h, width, height, refreshHz)
     }
 
+    /// Ask the host's encoder to emit a fresh IDR keyframe now — recovery when the local
+    /// decoder has wedged. The host opens the infinite-GOP stream with one IDR and then sends
+    /// P-frames only, so a stalled decode (a lost/corrupt opening IDR, a bad early P-frame —
+    /// most likely on the cold first connect) would otherwise stay frozen until the next
+    /// loss-triggered recovery keyframe, which may be far off. Fire-and-forget; the recovered
+    /// keyframe is the only ack. THROTTLE at the call site — the decode stays wedged for
+    /// several frames until the IDR lands, so requesting every frame would flood the control
+    /// stream. Silently dropped after close.
+    public func requestKeyframe() {
+        abiLock.lock()
+        defer { abiLock.unlock() }
+        guard let h = handle, !closeRequested else { return }
+        _ = punktfunk_connection_request_keyframe(h)
+    }
+
     /// The currently active session mode (updated by accepted `requestMode` switches).
     public func currentMode() -> (width: UInt32, height: UInt32, refreshHz: UInt32) {
         abiLock.lock()