phase 4: glass-to-glass via 16-byte stream header + /state stream stats

Wire format change: stream frames now carry a 4-byte "VPRT" magic +
4-byte jpeg_len + 4-byte seq + 4-byte event_us_low. Total 16 bytes
(was 8). The firmware sniffs the first 4 bytes per frame: if they
spell VPRT it reads the remaining 12 bytes of v1 header; otherwise
it interprets bytes 0-3 as jpeg_len for the old v0 8-byte format and
reads 4 more for seq. Lets a v1 firmware accept a v0 (legacy)
Scrypted script during the rollout window. v0 will be removed once
all field deployments roll forward.

event_us_low is the low 32 bits of the Scrypted host's monotonic µs
at camera-event arrival. The firmware does NOT interpret it (the
clocks aren't sync'd); it just stamps it on every painted frame and
exposes the most recent value via /state. The script polls /state
every 5s during an active stream, reads last_paint_event_us_low,
and computes glass-to-glass = (now_us_low - last_paint_event_us_low)
with 32-bit wrap. 30s sanity ceiling on the wrap to discard event
timestamps from before the stream started.

Also expose the firmware's just-closed 30-frame window stats via
/state under the "stream" key — frames, bytes, window_us, plus
min/avg/max for recv/dec/paint/idle. Lets external tools (a curl
loop, the Scrypted plugin, etc) poll the firmware's view without
parsing serial logs.

Firmware:
- stream_server.h: 16-byte v1 wire spec, stream_server_stats_t
  struct, stream_server_snapshot_stats(out) getter.
- stream_server.c: magic-detect header read path, last_event_us_low
  capture into per-connection state, portMUX-protected window-stats
  snapshot at every 30-frame roll.
- http_api.c: GET /state JSON gains a "stream" sub-object with the
  full snapshot.
- viewport_state.h: VIEWPORT_VERSION 1.0.0 → 1.1.0 (new /state shape).

Scrypted:
- startStream captures eventUsLow = (tEvent * 1000) >>> 0.
- TCP demux loop writes the 16-byte v1 header with the VPRT magic.
- New fwPoller setInterval (5s) fetches /state, parses .stream,
  computes g2g, emits one summary line per poll cycle.

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