Kura restores memory-mapped reads for small files on virtio-fs-backed storage

Kura’s mmap residency gate previously required every queried page, including the partial page past the end of a file, to be resident. On virtio-fs-backed storage, mincore reports that final partial page as non-resident even when its bytes are cached, so Kura fell back to the slower streaming reader for sub-page and unaligned-tail files. This fix exempts the final partial page from the residency check, restoring the memory-mapped fast path for small files on virtio-fs while still requiring every fully-backed page to be resident. The worst case is a single small read if the partial page is genuinely cold, so large artifacts are essentially unaffected.