perf(s3): coalesce the bulk-submit change log to one object per batch

[angela-helios]

Why

S3 $bulk-submit ingest wrote one change-log object per resource — one of the per-entry PUTs. With the raw archive already coalesced (#1411 / the re-land in #1433), that change-log PUT is the next-cheapest to drop: the change log has no per-object reader (rollback lists the whole changes/ prefix and sorts), so writing it once per batch instead of once per resource costs nothing on read. This takes the concurrent fast-load path from 6 → 5 PUTs/resource (#1429).

Stacked on #1433. Base is fix/945-reland-s3-ingest-guard-and-raw so the diff here is only the change-log change; GitHub retargets it to main when #1433 merges. Review/merge #1433 first.

What

• process_single_entry now returns (BulkEntryResult, Option<SubmissionChange>) instead of calling record_change — the batch gathers every change (from both the concurrent and serial paths) and writes them in one coalesced object via persist_change_batch, keyed by first line and discriminated by file_url, exactly as the raw archive and the entry receipts are (submit_change_batch_key).
• The coalesced write happens after the resources and receipts, before notify_batch_committed; its error is reported after the receipts and yields to the walk error (the ui: 1h "FHIR Resources over Time" chart stays on "Waiting for the live figures…" during a large import — dashboard blocks on storage scans instead of degrading to approximate data #1078 "report what was written" contract). Whatever changes a partially-failed batch produced are still written, so rollback coverage is preserved.
• load_changes reads both shapes under changes/: the coalesced array and a legacy single-change object — the rollback trait's record_change is untouched and still writes the single form for composite backends, so mixed submissions roll back fully.

Test

s3::tests bulk-submit suite green (29): two new — a three-entry batch records its changes in one object and list_changes reads all three back; load_changes reads a submission holding both a coalesced batch object and a legacy single-change object. The existing duplicate_abort_and_rollback already exercises coalesced-write + dual-format-read + rollback end-to-end. fmt + clippy clean under the CI allow-flags.

Perf

hfs-50 to A/B on MinIO (objects/resource ~6.0 → ~5.0, throughput +~12–15%, per #1429) and confirm rollback + dual-format correctness before this is considered validated.