Kura now serves small artifacts via mmap on virtio-fs-backed data directories

Kura’s memory-mapped file serving was gated on mincore residency, which on virtio-fs reports the final partial page (past EOF) as non-resident even when its data is cached. This caused sub-page or unaligned-tail artifacts to fall back to the streaming reader on Kata microVM / virtio-fs data directories. The residency gate now exempts that final partial page and only requires fully-backed pages to be resident. A new kura_mmap_partial_page_exemptions_total counter tracks exemptions, and the page-end arithmetic uses saturating operations to avoid overflow. The streaming fallback was always correct, so this is a performance/robustness fix, not a correctness change.