Implement CatalogClient class for catalog operations

Add CatalogClient for invoking CatalogInterface methods on VGI workers:
- Each method call spawns new worker (short-lived pattern)
- Simplified protocol: invoke → stream (no bind/init phases)
- All CatalogInterface methods mirrored with proper types

Core methods:
- catalogs() - List available catalogs
- catalog_attach/detach - Catalog lifecycle
- schemas(), schema_get/create/drop - Schema operations
- table_get/create/drop, table_scan_function_get - Table operations
- view_get/create/drop - View operations
- Transaction methods: begin/commit/rollback

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rustyconover