Backlog 4: the Flow image rebuild logged (refs #41) 7f75b41c107d

- 2026-10-01 00:00 **Second round** (6359f779 to 81a5bdd7; table in `docs/benchmarks.md`). Commit 0.67 s (from 0.96 s): the state index is brought up to date by replaying and rewritten only 16 changes behind; keys only for changed paths; attributes from the snapshot rather than a tree walk; the manifest checksummed and written without fsync; and a fix: after a status had seen an earlier edit, a commit kept that edit's stat in the manifest, so every later status took a full snapshot. Blame 0.16 s (from 0.59 s; Git 0.07 s), log 0.18 s, checkout 0.74 s each way (from 1.4 s) with two state indexes kept per workspace, and status right after a checkout that wrote 1 000 files 0.18 s (from 1.3 s). **R1** done: `oxy rebase --into-working-copy` (closes #42).

- 2026-10-01 00:40 **S6 done, clone level, P1 partly** (3287e4ff to cc1f1723). fsck 8.2 to 8.7 s (from 13.6 s; Git 12.6 s): objects checked a chunk at a time on every core, graphs built with their lines in parallel, and the last four change encodings kept rather than one (a batch that took inline text from the Linux tip and a later change in turn read the tip again at each switch). Local clone 16.8 to 17.6 s (from 32 s; Git 17.0 s): files added once and never edited are written from their text without a graph (test builds compare with the rendering), new files are written in parallel, rewritten ones one at a time (six times slower in parallel on ext4). A faster local clone would copy the source's packs as Git's `--local` does; that skips the import's checks and brings every object of the source, so it waits for Andy. **P1**: a trained zstd dictionary per pack: pack 293.5 MB (from 301 MB; Git's 295.5 MB), gc 33.7 s (Git 70 s). The whole `.oxy` stays larger than `.git` by the state index (52 MB, a cache; compressing it would slow every load).