fix(agent): make native H.264 viewer render live frames

The native viewer's H.264 path (Task 7 first-cut, compile-verified only)
never rendered a frame. Three stacked bugs, all confirmed via live loopback:

1. decoder: MF_E_NOTACCEPTING (0xC00D36B5) was treated as fatal and only
   one output was drained per call, so once the MFT filled it rejected
   every subsequent frame. decode() now returns Vec<DecodedFrame>, drains
   on back-pressure and retries the unconsumed sample, then drains all
   ready outputs.
2. decoder: the NV12 output type was hand-built and rejected by the MS
   H.264 decoder MFT (MF_E_TRANSFORM_TYPE_NOT_SET, 0xC00D6D60). It is now
   negotiated by enumerating GetOutputAvailableType on STREAM_CHANGE /
   TYPE_NOT_SET.
3. render: a manual pump_messages() in about_to_wait stole winit's own
   thread messages and froze the event loop after one iteration, so frames
   were never drained from the channel. Removed; winit's run_app pump
   already services the WH_KEYBOARD_LL hook.

Validated on a 5070 loopback: 0 decode errors, frames decode/paint/present
(present count 0 -> 1740). Reviewed (APPROVE-WITH-NITS); diagnostics stripped.

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

agent/src/viewer/mod.rs

@@ -74,20 +74,27 @@ fn spawn_h264_decode_worker(
 
                 let dec = decoder.as_mut().expect("decoder present after init");
                 match dec.decode(&data, pts) {
-                    Ok(Some(decoded)) => {
-                        let frame = render::FrameData {
-                            width: decoded.width,
-                            height: decoded.height,
-                            data: decoded.bgra,
-                            compressed: false, // already BGRA
-                            is_keyframe: false,
-                        };
-                        if viewer_tx.blocking_send(ViewerEvent::Frame(frame)).is_err() {
-                            // Viewer closed; stop the worker.
+                    // One input access unit may yield zero, one, or more frames.
+                    Ok(frames) => {
+                        let mut viewer_closed = false;
+                        for decoded in frames {
+                            let frame = render::FrameData {
+                                width: decoded.width,
+                                height: decoded.height,
+                                data: decoded.bgra,
+                                compressed: false, // already BGRA
+                                is_keyframe: false,
+                            };
+                            if viewer_tx.blocking_send(ViewerEvent::Frame(frame)).is_err() {
+                                // Viewer closed; stop the worker.
+                                viewer_closed = true;
+                                break;
+                            }
+                        }
+                        if viewer_closed {
                             break;
                         }
                     }
-                    Ok(None) => { /* decoder buffering; no output this tick */ }
                     Err(e) => {
                         warn!("H.264 decode error: {e:#}");
                     }