Kura serves small and unaligned-tail files through mmap again on virtio-fs-backed storage

Kura’s residency gate required every queried page, including the partial page that extends past a file’s end, to be in memory before serving a file through mmap. On virtio-fs, the filesystem used by Kata microVM runners, that final partial page is reported as non-resident even when its bytes are cached, so Kura fell back to the streaming reader for any file smaller than a page or with an unaligned tail. This release exempts that final partial page from the residency check: only pages fully backed by file data must be resident. Large artifacts are still gated page by page, so this restores the mmap fast path on virtio-fs-backed data directories without affecting correctness.