Problem

OpenOptions::page_cache_size is respected even when opening an in-memory database via open_memory(). The PageCache struct is allocated at full capacity (default 256 pages = 1MB), but all reads hit RAM directly — the cache is never useful and wastes heap.

Proposed fix

In Minigraf::in_memory_with_options, ignore page_cache_size and allocate a zero-capacity PageCache (or skip allocation entirely). The MemoryBackend does not go through the page cache path, so this is a safe no-op change with no semantic impact.

Acceptance criteria

• In-memory databases do not allocate the page cache regardless of page_cache_size.
• OpenOptions::page_cache_size is documented as file-backed only.
• Existing in-memory tests pass unchanged.