fix(adapters): recreate decode iterator after EAGAIN so stream resumes

[styu12]

Symptom

After PR #8 merged, the viewer's camera preview cards populate with the first frame only and then freeze — despite health events showing connected and no error events. The capture thread exits silently after the first EAGAIN retry cycle.

Root cause

PR #8 added EAGAIN handling but called next() on the same decode iterator after catching the exception. PyAV's container.decode() is a Python generator, and generators become permanently exhausted after any exception propagates out of them.

The flow was:

1. next(frame_iter) → first frame ✓ (camera warmed up)
2. next(frame_iter) → BlockingIOError(35) (between frames) — iterator dies
3. Handler: time.sleep(0.001); continue
4. next(frame_iter) → StopIteration (iterator is dead)
5. except StopIteration: break → capture thread exits silently, latest_frame frozen

Fix

Track frame_iter explicitly. Set it to None on every transient-errno retry, and recreate at the top of the next loop iteration:

frame_iter = None
while not self._stop_event.is_set():
    if frame_iter is None:
        frame_iter = iter(self._input.decode(video=0))
    try:
        frame = next(frame_iter)
        ...
    except OSError as exc:
        if exc.errno in _TRANSIENT_ERRNOS or exc.errno is None:
            frame_iter = None    # ← key change
            time.sleep(0.001)
            continue

PyAV's container.decode() returns a fresh generator per call that shares the underlying container's packet queue, so recreation resumes reading without losing session state.

StopIteration is now only hit on true end-of-stream (device disconnect) and exits cleanly.

Test plan

• 11/11 UVC unit tests pass (including the two TestDecoderResilience tests from PR fix(adapters): tolerate EAGAIN/EINTR in UVC capture loop #8)
• Manual smoke test with examples/iphone_mac_webcam/record.py — preview updates live at 30 fps, no freeze

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