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>

crates/punktfunk-core/src/abi.rs

@@ -1383,6 +1383,32 @@ pub unsafe extern "C" fn punktfunk_connection_request_mode(
     })
 }
 
+/// Ask the host's encoder to emit a fresh IDR keyframe now — client recovery when the
+/// decoder has stalled (the infinite-GOP stream sends one opening IDR then P-frames only, so
+/// a wedged decoder would otherwise freeze until the next loss-triggered recovery keyframe).
+/// Non-blocking, fire-and-forget; the recovered keyframe is the only ack. The caller should
+/// THROTTLE — the decode stays wedged for several frames until the IDR lands, so requesting
+/// every frame would flood the control stream.
+///
+/// # Safety
+/// `c` is a valid connection handle.
+#[cfg(feature = "quic")]
+#[no_mangle]
+pub unsafe extern "C" fn punktfunk_connection_request_keyframe(
+    c: *const PunktfunkConnection,
+) -> PunktfunkStatus {
+    guard(|| {
+        let c = match unsafe { c.as_ref() } {
+            Some(c) => c,
+            None => return PunktfunkStatus::NullPointer,
+        };
+        match c.inner.request_keyframe() {
+            Ok(()) => PunktfunkStatus::Ok,
+            Err(e) => e.status(),
+        }
+    })
+}
+
 /// A speed-test measurement, filled by [`punktfunk_connection_probe_result`]. `done` is 0 until
 /// the host's end-of-burst report lands, then 1 (the numbers are final). `throughput_kbps` is the
 /// measured goodput to drive a bitrate choice from; `loss_pct` is the delivery loss at that rate.