Measure real EnqueueFrame outcomes, then size the send window to fit RTT

Every "fix" for the mirroring freezes so far has been reasoned from code
rather than measured, because the one counter that could have falsified any
of them was blind by construction: `enqueue_frame` returns as soon as a frame
is *posted* to openscreen's TaskRunner, long before `Sender::EnqueueFrame`
decides whether to accept it. The frame pump's `enqueued_fps` therefore read a
healthy 30fps through every freeze.

Add `BreadcastEnqueueStats` (new FFI accessor, no behaviour change): per-second
counts of OK / MAX_DURATION_IN_FLIGHT / REACHED_ID_SPAN_LIMIT /
PAYLOAD_TOO_LARGE, plus the in-flight window gauges and RTT sampled at the
enqueue attempt, all surfaced on the existing "frame pump rate" line as
`accepted_fps` / `rejected_*`.

Measured against the real Chromecast, that settles it: 12.2% of frames were
being rejected with MAX_DURATION_IN_FLIGHT, in 85% of all seconds -- steady,
not just during visible freezes. Since breadcast enqueues already-encoded
frames, each rejection silently breaks the H.264 reference chain rather than
merely dropping a frame.

The measurement also corrects the diagnosis. The send window is
clamp(2*RTT, kMinSenderInFlight, target_playout_delay/3); the assumption was
that a LAN pins it to the 66ms floor. It does not -- RTT to this receiver runs
42-189ms, so 2*RTT is 84-378ms and the window was pinned at the *ceiling*,
133ms at a 400ms playout delay. The ceiling was the binding constraint, so
raising the floor alone would have changed nothing.

So raise both, ceiling first: target playout delay 400ms -> 1200ms (ceiling
133ms -> 400ms) and kMinSenderInFlight 66ms -> 200ms for RTT dips. Measured
over a matched 65s steady-state window, rejections fall 12.2% -> 4.3% and
seconds containing a broken reference chain 85% -> 40%. Costs ~800ms of
added latency, which is unnoticeable for mirroring to a TV.

This is an improvement, not a cure. The residual rejections are bursts
(in-flight seen at 433ms against a 200ms window, RTT spiking to 221ms), and no
static window survives those. The real fix is the backpressure contract
sender.h documents and this facade still doesn't implement: consult
GetInFlightMediaDuration()/GetMaxInFlightMediaDuration() and throttle *before*
encoding, so a skipped frame never leaves a dangling reference behind.

breadcast-core/src/caststream.rs

@@ -24,7 +24,7 @@ use breadcast_caststream_sys::{
     self as sys, breadcast_caststream_sender_create, breadcast_caststream_sender_destroy,
     breadcast_caststream_sender_enqueue_frame, breadcast_caststream_sender_estimated_bandwidth_bps,
     breadcast_caststream_sender_needs_key_frame, breadcast_caststream_sender_negotiate,
-    breadcast_caststream_sender_on_message,
+    breadcast_caststream_sender_on_message, breadcast_caststream_sender_take_stats,
 };
 
 /// The Cast Streaming ("Mirroring") receiver app id, pre-installed on every
@@ -236,6 +236,22 @@ impl CastStreamSender {
         Ok(())
     }
 
+    /// A snapshot of how the *underlying* `Sender::EnqueueFrame` has been
+    /// answering, plus its in-flight window gauges. Reading resets the
+    /// counters, so polling once a second yields per-second rates.
+    ///
+    /// [`Self::enqueue_frame`] deliberately cannot report any of this: it
+    /// returns as soon as the frame is posted to openscreen's TaskRunner,
+    /// before the real accept/reject happens. Any "frames enqueued per
+    /// second" figure derived from its return value is therefore a count of
+    /// *attempts*, and stays pinned at the capture rate even while every
+    /// frame is being rejected downstream and the picture is frozen.
+    pub fn enqueue_stats(&self) -> sys::EnqueueStats {
+        let mut stats = sys::EnqueueStats::default();
+        unsafe { breadcast_caststream_sender_take_stats(self.raw, &mut stats) };
+        stats
+    }
+
     /// True if the receiver wants a key frame as soon as possible. Cheap to
     /// poll frequently (e.g. once per captured frame, before encoding it).
     pub fn needs_key_frame(&self) -> bool {