Admission keyed on file size (>= 1 MB) instead of map length: the hot
set is the small repeated maps (pack trailer, idx fanout, delta bases)
that the 64 KB floor excluded — 0 hits across three real-repo runs.
Small mutable working-tree files stay uncacheable. p_munmap now evicts
unreferenced entries to a 2 MB low-water mark so released windows are
actually returned to git__malloc (fixes the 7.4 MB resident / 508 KB
heap zlib OOM and keeps MWINDOW_MAPPED_LIMIT honest).
dd3d70cbec