fix(host): encode-stall watchdog — heal the silent AMF/QSV freeze in place

Field reports: Windows AMD/Intel streams freeze after ~3-5 min regardless of
desktop activity. Root cause: the libavcodec AMF/QSV poll is non-blocking
(EAGAIN -> Ok(None)), and the encode loop's drain treated None as benign
without popping `inflight` — a wedged driver (QueryOutput stops producing)
meant frames kept being submitted, inflight grew unboundedly, no AU ever
reached the send thread, and nothing logged: a silent permanent freeze. The
input-side twin: once libavcodec's one-frame buffer fills, avcodec_send_frame
EAGAINs and the submit `?` killed the whole session.

Add `Encoder::reset()` (in-place encoder rebuild; implemented for AMF/QSV by
dropping the wedged libavcodec encoder so the next submit re-opens it on the
current device, forced IDR) and an encode-stall watchdog in the stream loop:
trip on a poll error, on no AU within max(2 s, 8 frame intervals) while frames
are owed, or on an owed backlog worth more than the window's frames (the
slow-leak latency-runaway form). Recovery is a bounded (5 consecutive, cleared
by any delivered AU) in-place rebuild + forced IDR — a logged ~one-second
hiccup instead of a dead stream; exhaustion or a reset-less backend still
fails the session with a clear error. Submit failures route through the same
bounded recovery. The three existing pipeline-rebuild paths (session switch,
mode switch, capture loss) now also clear the stale in-flight records that
pointed at the dropped encoder.

Backends whose poll blocks (direct NVENC sync, software) can't false-trip:
they never return Ok(None) mid-stream and drain inflight below depth each
tick. Validated: clippy -D warnings (nvenc,amf-qsv), 191 host tests, synthetic
E2E 300/300 frames, and an on-glass AMD iGPU session (1080p120 HDR hevc_amf).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-host/src/encode/windows/ffmpeg_win.rs

@@ -1307,6 +1307,18 @@ impl Encoder for FfmpegWinEncoder {
         self.force_kf = true;
     }
 
+    /// Encode-stall recovery: drop the wedged libavcodec encoder (its `Drop` releases the AMF/QSV
+    /// runtime state) and let the next `submit` rebuild it lazily on the current device, exactly
+    /// like first-frame bring-up. The owed AUs are forfeited (`in_flight` zeroed) and the rebuilt
+    /// encoder's first frame is forced IDR so the client resyncs immediately.
+    fn reset(&mut self) -> bool {
+        self.inner = None;
+        self.bound_device = 0;
+        self.in_flight = 0;
+        self.force_kf = true;
+        true
+    }
+
     /// Poll for the next finished AU (single non-blocking `receive_packet`).
     ///
     /// libavcodec's `hevc_amf`/`av1_amf` wrapper holds ~2 frames before releasing the oldest