unified end-to-end per-frame instrumentation + version stamps

Cross-side timing was opaque: script saw "req=70ms" but couldn't
split TCP/dispatch overhead from firmware decode time, and firmware
serial logs and Scrypted console logs couldn't be correlated.

Firmware /frame handler now emits Server-Timing on every response
with per-stage breakdown:
  Server-Timing: recv;dur=X, dec;dur=Y, paint;dur=Z,
                 post;dur=W, handle;dur=total
Script parses it, joins by X-Frame-Seq implicitly (one POST per seq),
derives net_up = req − fw_total, and logs a multi-line p50/p95
breakdown every 10 fetches:

  fetch "kitchen" #10 (jpeg=137KB)
     wall      p50=65ms  p95=140ms
     emit→post p50=0ms   p95=2ms      (queue wait)
     req       p50=62ms  p95=135ms    (fetch → Response headers)
       net_up  p50=43ms  p95=110ms    (TCP + body wire + dispatch)
       fw_recv p50=12ms  p95=18ms     (body off the wire)
       fw_dec  p50=6ms   p95=8ms      (hardware JPEG)
       fw_paint p50=0.1ms p95=0.2ms   (backbuffer flip)
       fw_post p50=0.4ms p95=0.6ms
     body-read p50=1ms                 (drain — empty body)
     inflight  d0=8 d1=2 d2=0
     stale-drops=0

Plus version stamps on both sides for the "is the user on the right
code" question:
- CMakeLists: PROJECT_VER = git short hash + -dirty marker if dirty.
  esp_app_get_description()->version exposes it at runtime. Boot
  log: "Scrypted Viewport boot (v0.1.0 build=4cf36e2-dirty)".
- TS: SCRIPT_VERSION const at the top, bumped per commit, logged at
  script-eval: "Scrypted Viewport up (script=4cf36e2). Callback ..."

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