Step LS-9: `blfile` reproduction

[craig-dt]

--as-of, --rebuild, collection_id, and the builder digests. Design is spec §6.5 and is not
re-derived. Note §4.1's limit: ingested_at does not survive a rebuild, so --as-of is an audit tool
across that boundary, not a reproduction one.

Issue #153 was originally "tier-2 ingestion"; that is now a gate rather than a step, tracked on #142
plus LS-8's --skip-tier.

The test that proves it — --rebuild reproduces over records with built_at excluded; a pinned
run_id missing from the store is a hard failure naming it; --rebuild --label and --rebuild --as-of
exit 2; a run whose finished_at precedes an --as-of cutoff but whose ingested_at follows it is
excluded — the assertion that pins the whole argument, and the one a plausible implementation gets
backwards.

Depends on LS-8.

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Revision 2 (2026-08-20). Phase 3b, step LS-9. Plan: docs/PLAN-label-store.md. Spec: docs/spec-label-store.md. The step ids did not change; the sequence and several file lists did — see PR #154.