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# Initial plan

## Order

1. MQTT client, subscribing to the retained vehicle topics. Text readout only —
   prove the contract before drawing anything.
2. Gauges.
3. Moving map: MapLibre against the PMTiles basemap served by the Orin, with the
   position from `yukart/gnss/fix`. No routing involved.
4. Guidance from `yukart/nav/guidance`, once [svc-nav](gitea:yukart/svc-nav) publishes it.

A moving map with position, heading and trace is most of the value in a car and
needs no routing engine at all. Routing is the expensive last fraction.

## Open

- **Transport.** Over USB the tablet has an IP link via RNDIS, so it can reach the
  broker directly. Whether it should, or should go through `svc-gw`, is decided in
  that repo — see its initial plan.
- **CJK labels.** MapLibre cannot ship Japanese glyph sets as SDF ranges. Local
  font rendering must be configured, or labels will be slow or absent.
- **Sunlight.** A typical tablet is unreadable through a windscreen in daylight.
  This is a hardware problem, but the UI should not make it worse.

## The boundary, from this side

Two channels with different guarantees, not one socket:

| | Down (state) | Up (commands) |
|---|---|---|
| Transport | push stream | request/response |
| Loss | tolerated; next sample supersedes | **not** tolerated; retry until acknowledged |
| On reconnect | expect current state immediately | resend anything pending |

This client always initiates and owns reconnection — the Orin never dials out.
Assume the link drops constantly: cable knocked, screen off, WiFi lost.

## Session marking is worth building early

A button that stamps an event (`indicator on`, `cold start`, `hard braking`) into
the recording, on the same clock as the CAN frames, turns the Phase 1 manual event
log from a notebook-and-wristwatch exercise into structured data. It is what makes
a raw capture decodable later, and it is a Phase 1 capability that happens to live
in this repo.