Journal::snapshot serialised the catalog into a String and then copied it into a second
`format!`-ed String before writing (2x the snapshot size in memory, ~620 MB at 85k photos).
It now streams `{"format":…,"seq":N,"catalog":` + serde_json::to_writer + `}\n` through a
1 MiB BufWriter into the store (new `Store::write_atomic_with`; FsStore writes the temp file
directly, then flush + sync_all + rename + dir fsync as before; other stores buffer, as before).
Durability and format are unchanged: same temp + fsync + atomic rename; a failed write leaves
the old snapshot (and now removes the temp file); the bytes are identical to the old format
(test `streamed_snapshot_keeps_the_on_disk_format`, which also loads a legacy snapshot), so
no version bump. Also tested: `failed_streamed_write_keeps_the_old_file`.
persist_bench, 85k photos (310 MB snapshot), compaction at the 2,000-record threshold,
before/after binaries alternated in the same session (load avg ~130-170), building+compacting
only (PERSIST_BENCH_NO_LOAD):
compaction median: before 3.5 s / 4.6 s, after 1.9 s / 2.5 s
peak RSS: before 926 MB / 1263 MB, after 308 MB / 366 MB
10k photos peak RSS: 149-235 MB -> 61-68 MB (times too noisy to compare)
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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