Data source: Kafka plugin

Streaming platform / message broker. **Non-SQL** — the most different paradigm.

Builds on the **Seshat** foundation (#93, shipped in #99): host `tcp-client` (+ host-side TLS), owner-thread query execution, `secure-storage` for credentials, and the `UiNode` DSL (connection dialog, dock tabs, results grid, `json-tree`).

## Transport
- Kafka binary wire protocol over TCP `:9092` (versioned request/response APIs). Auth: SASL (PLAIN / SCRAM), TLS.

## Reuse
- `tcp-client`, owner thread, `secure-storage`.

## New work (substantial)
- Kafka protocol: `ApiVersions`, `Metadata`, `Fetch`, `Produce`, `ListOffsets`, consumer-group APIs; SASL handshake. UI: brokers, topics/partitions, tail/consume messages, produce, consumer-group lag.

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Part of the data-source plugins epic #109.