feat: support empty directory markers

[andinux]

Summary

• Add explicit empty directory markers with SELECT memory_add_content('dirname/', ''); for virtual filesystem workflows.
• Require preserve_duplicate_paths=1 before marker creation so empty marker identity stays path-scoped.
• Store marker rows in dbmem_content with a trailing slash path and no vault/FTS rows, so search ignores them.
• Update listing, deletion, rename, materialization, directory cleanup, and reindex paths to handle markers consistently.
• Document the feature in README and API docs and add focused unit coverage.

Planned Choices

• Directory marker syntax is a trailing-slash path plus empty content: memory_add_content('dirname/', '').
• Marker creation returns a clear error unless preserve_duplicate_paths=1 is enabled.
• Non-empty content for a trailing-slash marker path is rejected.
• Marker rows are stored internally with one canonical trailing slash, for example dirname/.
• memory_list_files() renders marker rows as directory nodes and returns directory JSON paths without the trailing slash, for example "path":"dirname".
• A marker can coexist with files below it, for example dirname/ and dirname/file.md; the marker is retained so the empty directory can survive after children are removed.
• A file path and directory marker path with the same name conflict, so dirname and dirname/ cannot both exist as rows.
• Creating the same directory marker repeatedly is idempotent.
• Marker rows are metadata-only for search: no embeddings and no FTS rows are generated.
• memory_delete_file('dirname') and memory_delete_file('dirname/') delete only the explicit marker row; child files are not deleted recursively.
• memory_rename_file() supports marker-to-marker renames only when the new path also has a trailing slash, and it does not rename child paths.
• memory_materialize_files() creates directories for marker rows instead of writing empty files.
• memory_reindex() and model-change reindex preserve marker rows without indexing them.
• memory_add_directory() cleanup skips marker rows instead of treating them as missing files.
• Marker rows force their empty value to be stored even when save_content=0, because the value is marker metadata needed for materialization and sync behavior.

Verification

• make test TEST_DEFINES='-DSQLITE_CORE -DTEST_SQLITE_EXTENSION'