Fix mmap fast path for sub-page artifacts on virtio-fs-backed data directories

Kura now correctly uses memory-mapped serving for artifacts smaller than one page (or files with an unaligned tail) when the data directory is backed by virtio-fs. The residency gate previously rejected these files because mincore reports the partial past-EOF page as non-resident on virtio-fs, even when the cached bytes are present. Kura now exempts that final partial page from the residency check and only requires the fully-backed pages to be resident. If the partial page is genuinely cold, it may fault once as a single small read. The change also adds a kura_mmap_partial_page_exemptions_total counter so the frequency of these exemptions is observable in production.