Benchmarks: commit after one manifest write and parallel diffs; fresh store size against Git (refs #41) 4f264d6f20b6

### Commit: one manifest write, parallel diffs (2026-10-02, oxygen#41)

A commit's snapshot now hands its manifest rows to the commit, which

writes the manifest once with the new base's graph keys; before, the

snapshot wrote it and the commit read and rewrote it. The change builder

diffs edited text files in parallel before updating them one by one.

| | before | now | Git |

|---|---:|---:|---:|

| commit, one file, straight after the edit | 0.55 s | 0.34 to 0.45 s | 0.15 s |

| commit, 1 000 files | 0.99 s | 0.79 to 0.86 s | 1.1 to 1.2 s |

Of the one-file commit, about 135 ms is the operation's fsync: on ext4

with ordered data the first sync also writes the edited file's data.

Git's default commit does not sync loose objects. Of the 1 000-file

commit: building the change 72 ms (from 243), one read of the Linux

import's root change for the inline texts of about 90 of the files

(110 ms), rendering 97 ms, graph building 67 ms, the stat check 80 ms.

### Fresh store size and first commit (2026-10-02, P1 and S7)

`~/bench/p1.sh`: the Linux tip (95 942 files, 1.8 GB) committed fresh by

oxy and by Git in scratch copies, then gc.

| | oxy | Git |

|---|---:|---:|

| init and first commit | **25.2 s**, 3.1 GB peak | 30.8 s, 41 MB |

| objects after gc | **265 MB** | 285 MB |

| whole `.oxy` / `.git` | **278 MB** | 295 MB |

The bench repository's store is larger (290 MB of objects) because builds

before this one also stored the working-copy change a first commit's

snapshot made: an 85 MB change (21 MB packed) twice. A commit's snapshot

no longer stores it. The first commit's peak memory (S7, under 2 GB) is

still open: the walk holds every file's text.