MQTT subscription publish drops the delete/put `type` discriminator — deletes arrive as a bare null

## Summary

The MQTT publish path drops the `type` discriminator that the subscription listener already computed, so an MQTT subscriber cannot distinguish a **delete** from an **update whose value is null** — both arrive as a bare `null` payload. SSE and the REST WebSocket path forward the full envelope and are correct; the loss is MQTT-only.

## Mechanism (verified on `origin/main` @ `fb762a365`)

`server/DurableSubscriptionsSession.ts:352-358` forwards only `update.value` (plus `update.version` separately) to the MQTT wire listener:

```js
const result = await this.listener(
resourcePath + '/' + path,
update.value, // <-- update.type is never passed
messageId,
subscriptionRequest,
update.version
);
```

`resources/Table.ts`'s `subscribe()` listener has already computed `type` (`put` / `delete` / `invalidate`) by this point — it is discarded here rather than being unavailable.

Contrast `server/REST.ts:504-508` (the WebSocket path), which passes the whole `result.value` envelope — `type` included — into `serializeMessage()`.

## Blast radius

MQTT subscribers only. Every delete on every subscribed table. A subscriber maintaining a local materialized view has no way to tell "this record was deleted" from "this record's value is now null", so it either keeps deleted records or drops null-valued ones.

## Suggested fix

Forward the discriminator on the MQTT path the way the WS path does, rather than reconstructing it from a null value at the client. The fix site is the single call above.

*Found by the qa-explorer QA loop (finding F-181); dup-searched across `harper` and `harper-pro` in all states with six phrasings — nothing tracks this today.*

Start in server/DurableSubscriptionsSession.ts:352-358, then inspect the subscription listener in resources/Table.ts and the contrasting WebSocket path in server/REST.ts:504-508. Verify the MQTT publish path preserves the computed put/delete/invalidate discriminator, and test that deletes remain distinguishable from updates whose value is null.

Written by the indexing model from the issue text.