Restore mmap fast path for small files on virtio-fs

Kura’s memory-mapped file serving no longer needlessly falls back to the streaming reader for sub-page or unaligned-tail files on virtio-fs-backed data directories. The residency gate used to require every queried page (including the file’s final past-EOF partial page) to report resident. On virtio-fs, mincore reports that partial page as non-resident even when its bytes are cached, so the gate returned a negative result and streamed instead. Now only pages fully backed by file data must be resident, exempting the final partial page. This restores the mmap fast path for small files (e.g., 16-byte or 23-byte files) on virtio-fs-backed runners such as the Kata microVM tuist-linux CI environment, while large artifacts remain gated strictly.