Restore mmap fast path for small and unaligned artifacts on virtio-fs

Kura’s mmap serving no longer falls back to the streaming reader for sub-page files or unaligned tail regions when the data directory is on virtio-fs. The residency gate now only requires fully-backed pages to be resident, and exempts the final partial page that extends past EOF. This fixes the regression where small artifacts were served slowly on virtio-fs-backed volumes, while large artifacts remain strictly gated as before. The change also includes a kura_mmap_partial_page_exemptions_total counter that records when the exemption is what allows an mmap serve to proceed.