Add the preserve_duplicate_paths option for virtual-file/editor workflows that need distinct logical paths even when content is identical or empty.

When enabled with SELECT memory_set_option('preserve_duplicate_paths', 1), storage hashes are scoped by path so dbmem_content can keep separate rows while the embedding cache still reuses chunk embeddings by text.

Fix empty content handling so memory_add_content() and memory_add_file() can store zero-length entries without producing chunks, and keep default deduplication behavior unchanged when the option is 0.

Document the option, bump the extension version to 1.3.2, and cover default dedupe, duplicate preservation, and empty file/content behavior with unit tests.