Expose cliff sensor data via /api-sdk/{begin,get,stop}_cliff_stream

[KuriGohan-Kamehameha]

Summary

Adds three HTTP endpoints to the SDK app that let callers poll Vector's cliff-detection state, mirroring the existing begin_event_stream / get_stim_status / stop_event_stream pattern. This is purely additive — no existing endpoint or behavior is touched.

User need

Open-loop motor control via wire-pod (drive_wheels, move_wheels, lua scripts that issue raw chassis commands) is unsafe near table edges because there is no SDK-side way to check whether the bot is currently sensing a cliff. Today every consumer that wants to gate movement on cliff state has to write its own gRPC client, duplicate auth, and join the event stream. These three endpoints let any HTTP client do the same with a single curl.

Implementation

• New event-stream goroutine subscribes to the robot_state event type (the only place DDL's external gRPC interface exposes cliff data; there is no cliff_event and no raw per-sensor field in RobotState).
• The aggregate ROBOT_STATUS_CLIFF_DETECTED bit (16384) is decoded from RobotState.Status on each event and cached on the Robot struct.
• get_cliff_status returns JSON shaped {sensors[4], detected[4], any_detected, status, stamp_ms}. The arrays are length 4 to match the physical sensor layout (FL/FR/BL/BR) but currently mirror the aggregate bit, since DDL's external interface does not expose per-sensor raw values. The raw status bitmask is included verbatim for callers that want it.

New endpoints

endpoint	response
GET /api-sdk/begin_cliff_stream?serial=ESN	done
GET /api-sdk/get_cliff_status?serial=ESN	JSON or error: must start cliff stream
GET /api-sdk/stop_cliff_stream?serial=ESN	done

Smoke test

Built on real escapepod hardware (Go 1.19.4, Pi 4, vosk STT) and tested against a paired Vector at serial 00509655:

=== conn_test ===
success
=== get_cliff_status before begin ===
error: must start cliff stream
=== begin_cliff_stream ===
done
=== get_cliff_status (after 2s) ===
{"sensors":[0,0,0,0],"detected":[false,false,false,false],"any_detected":false,"status":1056512,"stamp_ms":1778543227694}
=== get_cliff_status (after 3s) ===
{"sensors":[0,0,0,0],"detected":[false,false,false,false],"any_detected":false,"status":1056512,"stamp_ms":1778543230780}
=== stop_cliff_stream ===
done
=== get_cliff_status after stop ===
error: must start cliff stream

Five consecutive polls at 1Hz showed stamp_ms advancing in real time (1778543246537 → 1778543250923) with the bot parked on the charger, confirming the goroutine is live-decoding events rather than returning stale state. status: 1056512 lacks the CLIFF_DETECTED bit (16384), so any_detected: false is correct for the parked-on-charger condition.

Test plan

• Patch builds cleanly with the existing chipper toolchain (vosk path, nolibopusfile tag)
• begin_cliff_stream returns done and spawns one goroutine per serial
• get_cliff_status returns the documented JSON shape with live stamp_ms
• get_cliff_status returns error: must start cliff stream when not started, including after stop_cliff_stream
• stop_cliff_stream returns done and clears cached state
• No regression: existing /api-sdk/* endpoints (conn_test, etc.) still work
• Verify any_detected flips to true when the bot is physically lifted off a surface (reviewer-side, requires hands on hardware)

Notes

This change does not touch any existing code path; the patch is purely additive (one new field block on Robot, three new cases in the handler switch, one extra flag-reset in removeRobot). 89 lines added, 0 removed.

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