Implement Ito MVP vertical slice

[arteeh]

Motivation

• Deliver the smallest working vertical slice described in mvp.md: a WebXR client, a recorded (mock) robot driver, and a mock visual processor that connect via direct WebRTC.
• The original client and drivers/mock-robot submodules could not be fetched in this environment, so a vendored, in-tree implementation is required to produce a reviewable, runnable MVP.
• Keep the first-version protocol minimal: GET / descriptor, POST /webrtc SDP signaling, video on media tracks, and JSON messages on control/status/scene DataChannels.

Description

• Add a vanilla WebXR client in client/ using Three.js and PLYLoader that discovers descriptors, offers raw and processed modes, creates control/status/scene DataChannels, streams pose updates at ~30 Hz, and reassembles/renders PLY scenes received from the processor.
• Implement the recorded robot driver in drivers/mock-robot/ using FastAPI and aiortc, looping a mounted video via MediaPlayer and MediaRelay, exposing a descriptor on GET /, accepting offers on POST /webrtc, accepting control/status channels, validating control sequence/enabled, and running a 500 ms watchdog that triggers safe_stop with a stop acknowledgement over status.
• Implement the mock mono processor in processors/mock-mono/ using FastAPI and aiortc, exposing a descriptor, accepting the client POST /webrtc, accepting driver video-source info over status, establishing a direct recv-only connection to the driver, counting frames to report ready/stale, validating and splitting the mounted PLY into safe-sized chunks, and sending a JSON header + sequential binary chunks over a reliable scene channel with support for a resend-scene request.
• Add root orchestration and developer conveniences: compose.yaml (host networking to surface WSL host ICE candidates), client HTTPS dev hosting via checked-in startup cert, .env.example, tests/compose-smoke.sh, component requirements.txt and Dockerfiles, and a PR log at pr-logs/2026-06-12-mvp-vertical-slice.md documenting implementation decisions.

Testing

• python -m py_compile drivers/mock-robot/app/*.py processors/mock-mono/app/*.py — succeeded.
• (cd drivers/mock-robot && pytest -q tests/test_control.py) — 2 passed.
• (cd processors/mock-mono && pytest -q tests/test_scene.py) — 2 passed.
• node --test client/tests/protocol.test.js — 1 passed.
• node --check client/src/main.js && node --check client/src/protocol.js — syntax checks succeeded.
• Environment-limited actions that could not be executed here: git submodule update --init --recursive failed due to blocked GitHub access (HTTP 403), npm install failed due to registry access restrictions (HTTP 403), and Docker/docker compose up could not be run in this environment; these limitations and the related implementation decisions are recorded in pr-logs/2026-06-12-mvp-vertical-slice.md.

---

Codex Task