[groupsky]

Every bus still on the old reader writes to a device over MQTT, so none of them can move to the bridge until it can write.

What it does

• Option: allowWrites: true on a device (default false). Without it, every write message gets an error result and nothing is written, so a meter is not written by mistake.
• Write topic: {topicPrefix}/{deviceId}/set, one per enabled device. Payload: a JSON object { "<dataPointId>": value, ... }.
• Checks first: the device allows writes, and every point exists and is writable. If one fails, nothing is written.
• Same turn on the bus as the polls: the writes and the read after them run as one job in the per-connection queue of the polling scheduler. So a write never runs in the middle of a poll, and the transport adds the same pause between devices.
• Order: points are written in message order. A driver error stops the writes: the points after it are not written; the points before it stay written, and the result lists them in written.
• Read at once: after the writes (also after a driver error, so the real state shows), the bridge reads the device and publishes on {topicPrefix}/{deviceId}/data. The read covers the points a poll reads. A failed read here is left to the next poll.
• Result: on {topicPrefix}/{deviceId}/set/result, QoS 0, not retained: { deviceId, timestamp, ok: true, values } or { deviceId, timestamp, ok: false, error, values, written }. The error names the point and why; written is [] when nothing was written.
• Retained messages: the stored copy that the broker sends again at each subscribe is ignored (with a warning), so an old command is not written again at every start or reconnect.

All of it is in the mqtt-bridge README, section "Writing Data Points".

Notes

• The issue says a value the driver rejects means "the other points are not written". The driver interface has no way to check a value without writing it, so the points before the failing one stay written. The result lists them in written, and the README says so.
• A written point with pollType: 'static' is not read again; /data keeps the kept value. No driver has a writable static point.

Tests

• Unit (index.writes.test.ts, polling-scheduler.writes.test.ts): one point; several points in order; unknown point; read-only point; device without allowWrites; driver error (also on the first point), with written and the /data after the partial write; read and publish right after the write; a write waits for a running poll on the same port; retained message; invalid payloads.
• End-to-end (write.integration.test.ts): real broker and MQTT client, real transport, TCP emulator. Writes a register over /set, sees the new value on /data and ok: true on the result.

Refs #675

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