Reuse persisted provider/model settings by creating missing local, remote, or custom engines on first embedding use instead of requiring memory_set_model on every connection.

Keep memory_set_model eager so callers can preload and validate engines explicitly, while memory_set_apikey remains connection-scoped and lazy for saved remote models.

Update API/README documentation and add regression coverage for saved local, remote, and custom provider settings.

Verification: build/unittest with TEST_SQLITE_EXTENSION passed 157 tests.

Keep the stored-hash duplicate check active when preserve_duplicate_paths is enabled so adding the same path/content tuple twice remains a no-op instead of hitting the unique path constraint.

Different paths with identical content still receive path-scoped hashes and remain preserved. Added a regression test for repeating memory_add_content('path/api4.md', '') with preserve_duplicate_paths=1.

Add explicit empty directory markers via memory_add_content('dirname/', '') for path-preserving virtual filesystem workflows. Directory marker creation now requires preserve_duplicate_paths=1, stores marker paths with a trailing slash, rejects file/directory conflicts, and keeps markers out of search indexes.

Update listing, delete, rename, materialize, directory sync cleanup, and reindex paths to handle markers consistently. Document the new behavior and add focused unit coverage for marker creation, conflicts, listing, deletion, materialization, and reindex survival.

- feat: lazily initialize engines from saved settings (#10)
- fix: keep preserved duplicate paths idempotent
- feat: support empty directory markers (#11)