Data source: MongoDB plugin

Document database. **Non-SQL** — new adapter shape + browsing 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
- MongoDB wire protocol (`OP_MSG`) over TCP `:27017`, BSON payloads. Auth: SCRAM-SHA-256 (same family as the Postgres SCRAM already implemented), TLS.

## Reuse
- `tcp-client`, owner thread, `secure-storage`, tabs, and `json-tree` for documents.

## New work
- MongoDB wire protocol + BSON codec; SCRAM-SHA-256 auth; a document browser (database → collection → documents), a find/filter query editor (JSON), and results rendered as a `json-tree` list rather than a table.

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Part of the data-source plugins epic #109.