Fix silent frame-chain corruption on EnqueueFrame rejection; revert to 720p

Root cause (found by Opus 5 second-opinion review) of the freezing that
survived every prior fix tonight: openscreen's Sender caps in-flight
unacknowledged media at clamp(2*RTT, 66ms, 133ms) -- on a LAN that's
pinned at the 66ms floor, about two frames at 30fps. When EnqueueFrame
rejects a frame under that budget, facade.cc discarded the result
(`(void)video_sender->EnqueueFrame(frame)`) and moved on. But
vah264enc had already encoded the *next* frame as a P-slice depending
on the one that just got silently dropped -- the encoder has no idea
the drop happened, since it happens downstream of encoding, at this
FFI boundary. The receiver sees an unbroken frame-ID sequence (nothing
here told it otherwise) and decodes a P-slice against a reference
picture it never received: a stuck/corrupted frame until the next
regularly-scheduled key frame (up to ~2s, longer if that key frame is
itself dropped the same way -- explains the 20+s outlier). Zero
"receiver reported picture loss" lines across 195s of a visibly
freezing session confirms the receiver genuinely never noticed
anything was wrong, which a real decode-capability or packet-loss
problem would have triggered.

This also explains why moving to a faster network made it *worse*:
the in-flight budget is RTT-derived, not bandwidth-derived, so more
throughput doesn't raise the 66ms floor at all -- while 1080p tripled
the per-frame packet count, increasing how often frames missed that
window.

Fixed the actual corruption: both drop paths in facade.cc (the
EnqueueFrame rejection, and the pre-existing non-monotonic-capture-time
guard) now set a `frame_chain_broken` flag, consumed once by
breadcast_caststream_sender_needs_key_frame() so frame_pump_loop forces
a key frame on the very next frame instead of chaining more P-slices
onto a reference that no longer exists on the receiver. A separate flag
from the existing `needs_key_frame` atomic because SchedulePoll's 100ms
timer unconditionally overwrites that one with the Sender's own
(unrelated) NeedsKeyFrame() reading, which would have silently clobbered
this signal.

Also reverted the Cast Streaming pipeline from tonight's 1080p
experiment back to 1280x720 (build_video_pipeline_for_streaming +
VideoParams::default(), which must agree -- a mismatch there is a
separate protocol-level bug fixed earlier tonight). Not the root cause,
but a real contributing factor per the packet-count reasoning above, and
untangling it from the frame_chain_broken fix by changing both at once
would make the next test ambiguous. Kept the 6000/1500 kbps bitrate
range from earlier tonight, now actually paired with 720p for the first
time.

The deeper real fix -- raising openscreen's 66ms in-flight floor itself,
which trades latency for headroom -- is out of scope for tonight; this
targets the corruption mechanism (via the sanctioned, if awkward,
needs_key_frame signal) without touching vendored openscreen constants.

breadcast-core/src/caststream.rs

@@ -55,12 +55,14 @@ impl Default for VideoParams {
             // a resolution other than what's actually sent is a real
             // protocol mismatch that plausibly explains a receiver decoder
             // corrupting/freezing rather than just looking soft.
-            width: 1920,
-            height: 1080,
+            // Reverted from a brief 1920x1080 experiment -- see
+            // `build_video_pipeline_for_streaming`'s doc comment for why
+            // (the freeze wasn't a resolution/bandwidth problem at all).
+            width: 1280,
+            height: 720,
             // Kept equal to `breadcastd::cast_mirror::MAX_BITRATE_KBPS *
             // 1000` -- see that constant's doc comment for why 8 Mbps
-            // (this struct's previous value) isn't used here even though
-            // 1080p can look better with more headroom: real hardware
+            // (this struct's previous value) isn't used here: real hardware
             // testing showed the AIMD probe pinning to whatever this
             // ceiling is for the entire session once the estimator reports
             // (unreliably) that there's room, and 8 Mbps sustained was more