scrypted: skip-oldest backpressure on stream socket

Hold at most one pending frame; new ffmpeg frames arriving while the
socket is backpressured replace the held one (drop oldest) instead of
queueing. On 'drain', flush the held frame. The in-flight write stays
out of our hands. Steady-state Node-side buffer drops from up to 20MB
to ~1 frame, eliminating the cap-and-reconnect loop as the primary
shedding mechanism — it's now an emergency safety net for stuck
connections that never fire 'drain'.

Capture-time timestamp instead of write-time so a held frame keeps its
true age and g2g reflects any pending-slot hold latency.

New log fields: drop-oldest=N (frames shed from the pending slot this
window) and bp=N% (share of ffmpeg frames in the window that found the
link backpressured at decision time).

0 files changed, 0 insertions, 0 deletions