p_mmap emulated a mapping by malloc+read()ing the range from SD on every
call. libgit2 re-hits the pack idx/windows on every object write (via
git_odb__freshen -> git_odb_refresh), so a commit re-read pack bytes over
SPI repeatedly -- ~500ms-1.1s/op on a small repo, far worse on the real
570 MB pack.
Cache read-only mappings >= 64 KB (pack idx/windows, commit-graph, midx,
packed-refs -- all immutable on this device) in a refcounted, PSRAM-backed
slot table keyed by (dev, ino, size, mtime, offset, len), LRU-evicted under
a soft cap. Small mutable working-tree maps (diff_file.c) fall below the
floor and are never served stale.
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