Catalog needs a database-identity contract for early cross-catalog validation

[shcheklein]

Split out of #1863 (see its review discussion).

DatasetQuery.union/join/subtract validate only Catalog.in_memory (_ensure_same_catalog_mode). Combinations across catalogs that live in different databases but share a mode are accepted and fail much later, deep in SQL execution, with a misleading missing-table error:

• two persistent SQLite catalogs backed by different db files
• (post-fix: honor in_memory everywhere, plugging a throwaway SQLite catalog #1863) any future design that gives in-memory catalogs distinct named databases

Proposal:

• add Catalog.database_identity (or is_compatible_with(other)) derived from the execution warehouse/database — for SQLite, the normalized db file or shared-memory identity; for server warehouses, the connection identity
• use it from union, join, subtract instead of the mode flag
• cover: two different persistent SQLite files (reject early), the same database through two sessions (allowed), distinct in-memory identities (reject early)

This gap predates #1863 for persistent catalogs; the mode check added there intentionally covers only the mixed in-memory/persistent case.

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