scrypted: max_node_buf_mb cap → destroy+reconnect socket to drop entire backlog

User's intent: "we want to see what's going on right now, not all the
shit we throttled and backlogged in the buffer." Dropping new arrivals
when the buffer is full doesn't help — the already-queued frames still
get processed in order, painting 5+ seconds of stale content before
catching up to live.

Better: when the buffer exceeds the configured cap, blow away the
whole queue by destroying the socket. The firmware's accept_task
picks up the next reconnect within ~500ms and the pipeline restarts
with the freshest ffmpeg frame as seq=1. Trade a sub-second visual
gap for clearing the entire backlog instantly.

Per-viewport setting "Max Scrypted-side buffer (MB)" under Display.
Default 20 MB (≈5s buffering ceiling at our measured ~3.5 MB/s
ffmpeg → firmware throughput). Range 1-200. Lower for tighter g2g
at cost of more frequent reconnect gaps under sustained backpressure;
higher to tolerate longer backlogs before flushing.

flushCount tracked per-stream and surfaced in the unified log line
alongside drops, plus node_buf shown vs the cap so the user can see
how close they're running to the threshold.

Demux loop now: if !socketReady → drop. Else if writableLength >
cap*MB → log + sock.destroy() + increment flushes. Else normal write.

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