feat(bench): attribute the loose-write residual — dir-scaling + splice I/O counters

sd_bench grows a directory-entry-scaling section (stat hit/miss and
the loose-object composite at 8/64/256 siblings): FAT resolves every
path component by linear scan, so cost climbing with N convicts
directory scans as the ~360ms residual; flat cost exonerates them.
stage_and_commit brackets the splice with the p_mmap counters and logs
'N mmaps / M KB read' per commit, splitting the residual into pack-read
I/O vs everything else on the next real :sync.

docs/v0.7-search-and-git.md

@@ -51,9 +51,15 @@ Sync performance — inherited from the
 - [ ] Close the publish gap: 19.8 s today — the splice's 5.0 s and the ~7 s of
       repo-open/negotiation overhead are the next curves
       ([tradeoff analysis](tradeoff-curves/sync-commit-staging.md)).
-- [ ] Instrument the residual ~360 ms/loose-write (suspect: FAT directory-op
-      cost in the freshen/refresh path) — one `sd_bench` + `p_mmap`-miss
-      logging pass.
+- [~] Instrument the residual ~360 ms/loose-write (suspect: FAT directory-op
+      cost in the freshen/refresh path) — **instrumentation BUILT 2026-07-14,
+      measurement run pending**: `sd_bench` grew a directory-entry-scaling
+      section (stat hit/miss + the loose-object composite at 8/64/256
+      siblings — FAT resolves paths by linear scan, so cost climbing with N
+      convicts dir scans; flash with `just flash-bench`), and `stage_and_commit`
+      now brackets the splice with the `p_mmap` counters, logging
+      `N mmaps / M KB read` per commit so the next real `:sync` splits the
+      residual into pack-read I/O vs everything else.
 - [ ] The images-off-card lever
       ([notes/git-sync-images-and-repo-size.md](notes/git-sync-images-and-repo-size.md))
       composes with the splice — decide whether to take it here.