M2: HTTP server + mDNS service discovery + shared state

- viewport_state.{h,c}: shared state struct (config, run-state, counters,
  orientation, timestamps) behind a FreeRTOS mutex. Modules update it
  under viewport_state_lock(); GET /state serializes a snapshot.
- http_api.{h,c}: starts esp_http_server on :80, registers GET /state.
  Returns the full JSON shape from the spec (name, version, configured,
  state, uptime_ms, last_frame_ms_ago, frames_received, decode_errors,
  state_post_failures, resolution, ip, free_heap, free_psram).
- mdns_service.{h,c}: mdns_init + advertise _scrypted-viewport._tcp.local
  on :80 with version/resolution/orientation/name TXT records.
  mdns_service_refresh() reapplies hostname + TXT after /config writes
  (called from M4 onward).
- main/CMakeLists.txt: add esp_http_server, esp_timer, json, mdns to
  REQUIRES. espressif/mdns added as managed component via
  main/idf_component.yml.

app_main calls viewport_state_init early so any module can read defaults
before /config arrives.

Also adds TESTING.md tracking per-milestone verification status and an
integration test plan to run after M9 (races, failure modes, longevity,
power cycles, negative protocol, multi-viewport).

M1 promoted to "🟡 builds clean" in TESTING.md; combined HW verification
of M1+M2 will be one flash session. Build verified against ESP-IDF 5.4
for target esp32p4 (binary ~550 KB).

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