Kura restores mmap serving for sub-page files on virtio-fs

Kura’s memory-mapped file serving now works correctly on virtio-fs-backed data directories for sub-page and unaligned-tail files. Previously, the mmap residency gate treated the partial page past EOF as requiring resident status; on virtio-fs, mincore reports that page as non-resident even when its bytes are cached, which needlessly disabled the mmap fast path and forced a fallback to the streaming reader for any sub-page file. The gate now only requires fully-backed pages to be resident and exempts the final partial page, restoring the mmap serving path on virtio-fs while keeping strict gating for every fully-backed page on native filesystems.