docs: merge Creality RETRO.md into PROTOCOL.md — single authoritative agent skill

Merged real hardware observations (SSH live, Ender-3 V4) with Flutter source
extraction into one document. Key additions vs Flutter-only version:

- Heartbeat {"ModeCode":"heart_beat"} → must reply "ok" (raw string, NOT JSON)
  — previously absent, causes silent disconnection if missing
- pause/stop/lightSw set commands (incorrectly marked "unknown" in Flutter doc)
- Moonraker HTTP port 7125 (start print / delete file)
- getGcodeFileInfo2 response with real encodings (nozzleTemp ×100, bedTemp ×100,
  timeCost in seconds, materialColors, filamentWeight, match field)
- Thumbnail routes: /downloads/humbnail/ (firmware typo, no 't') +
  current_print_image.png priority logic (pre-sliced > live fallback)
- type:1 = EXT slot, type:0 = CFS module (from real hardware, not Flutter guess)
- Camera: both webview approach and Tiger Studio RTCPeerConnection approach documented
- Push behavior: only mini-blobs (nozzleTemp+bedTemp0) after init burst

Contradictions resolved: boxsInfo.type ground truth from real hardware.
RETRO.md preserved as historical reference.

Author: TigerTag-Project