feat(explorer): add optional schema-first hierarchy

[Maxteabag]

Adds an optional Database → Schema → object type explorer layout for PostgreSQL/Supabase, SQL Server and Snowflake. Object-type grouping remains the default; SQLite and providers without the capability keep their existing layout.

Use :explorer for the picker, or :explorer schema / :explorer type. The preference persists, query text is preserved, and each layout retains its own expanded branches.

Shortcut suggestion: Space, then g (“grouping”) to toggle the two layouts in Normal mode. This is a proposal; that shortcut is not bound in this draft.

Closes #166.

Validation

• 66 provider-lane checks passed, zero skipped: real PostgreSQL and SQL Server containers, local SQLite, Supabase's actual adapter on local PostgreSQL transport, Snowflake emulation with fakesnow, plus routing contracts for all 31 registered providers.
• 1,750 unit/UI tests passed; 3 skipped.
• TDD red/green regressions fixed SQL Server index DDL dropping its schema and Snowflake catalog loading. Enabled object folders and PostgreSQL/SQL Server worker metadata are exercised.
• Provider integration CI passed on this head; JUnit reports and app captures are retained as artifacts.
• Known warning: the UI capture emits an existing column-prefetch coroutine-cleanup warning. No claim of live Snowflake or Supabase SaaS validation; the Snowflake emulator does not prove nonempty procedure/sequence behavior.

Provider implementation screenshots

Actual app captures at 140 × 44 terminal cells, after schema-qualified fixture queries. Expand each image; titles identify live engines, local transport, or emulation.

Mssql — live SQL Server 2022 container

Postgresql — live PostgreSQL 16 container

Snowflake — fakesnow emulation; not live Snowflake

Sqlite — local SQLite file

Supabase — real adapter with local PostgreSQL transport; not live Supabase