Data source: ClickHouse plugin

Columnar OLAP database. **SQL** — fits the existing `DbAdapter` pattern closely.

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
- HTTP interface on `:8123` (simplest to start) **or** the native binary protocol on `:9000`. Auth: user/password, TLS.

## Reuse
- `DbAdapter` trait, results grid, schema browser, EXPLAIN view.

## New work
- HTTP (or native-protocol) client over the shim; ClickHouse type mapping (columnar/nullable/array types); introspection via `SHOW DATABASES/TABLES` + `DESCRIBE`; `EXPLAIN` / `EXPLAIN PIPELINE`.

Depends on the shared **host HTTP client** prerequisite if using the HTTP interface (see umbrella).

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Part of the data-source plugins epic #109.

Read the Seshat foundation from #93 and #99, then trace the existing DbAdapter trait, results grid, schema browser, and EXPLAIN view. Resolve the shared host HTTP client prerequisite before choosing HTTP on :8123 or the native protocol on :9000; done means authenticated ClickHouse queries, type mapping, introspection, and EXPLAIN/EXPLAIN PIPELINE work through the plugin.

索引モデルが issue の本文から書いたものです。