feat: add session adapter authoring primitives

[roackb2]

Summary

• export a strict, database-neutral ChatSessionPersistenceCodec for complete opaque records and catalog projection
• export canonical ChatSessionCatalogPagination ordering, cursor, and page-limit behavior
• refactor the file adapter to consume those projection/pagination primitives while preserving tolerant legacy file reads
• document the host/database boundary and add focused public-API coverage

Why

The first production database adapter had to duplicate Heddle's record validation, catalog projection, stable ordering, cursor encoding, and page validation. This slice makes those session-domain mechanics reusable without adding a database, ORM, tenancy, or RLS dependency to Heddle core.

The strict remote-record parser remains separate from the compatibility-oriented file codec. During verification this also exposed and fixed an undeclared revision property leaking from file catalog metadata into returned ChatSession values.

Deliberate boundary

This PR does not add a PostgreSQL/MySQL adapter or conformance runner. Hosts still own SQL, migrations, transactions, connection pooling, identity scope, tenant filters, RLS, and database error translation. A reusable repository conformance kit is the next slice; async archive persistence comes before claiming full multi-replica durability.

Verification

• focused adapter/file tests: 21 passed
• full unit suite: 694 passed
• full integration suite: 295 passed
• yarn typecheck
• yarn lint
• yarn build
• built-package public export smoke