[simone-chen]

Problem and change

• An interrupted snapshot can retain a CSV row before its callback reaches the checkpoint. Resume replays that callback; first finalization previously persisted both measurements when no Parquet target existed. Apply the existing latest-row policy on first finalization while preserving raw evidence and current-row attestation.
• Pandas 3 can widen Arrow string keys to large_string during a compatible merge. The next merge then mistakes its own output for an incompatible dataset and replaces earlier rows. Preserve the validated, metric-reconciled Arrow schema and metadata through conversion.
• Keep measurement identities, newest-attempt selection, MoE eligibility inheritance, metric reconciliation, and genuine incoming schema-mismatch behavior unchanged. Explicit merge_existing=false still overwrites.

Validation

• 60 finalization tests passed with pandas 3.0.2 and PyArrow 24.0.0. New regressions cover three repeated merges with disjoint/replaced keys, nullable identities, both string widths, field/schema metadata, and strict incompatible-type handling. Both schema-preservation tests fail before this fix.
• 270 provenance recovery tests passed. Ruff lint/format and diff checks passed.
• The original 12-row MSA CSV remains 12 → 12 → 12 through the public finalizer with merging enabled, including a final one-row update. Direct merges also retain all rows for both string widths and keep a deliberately slower latest measurement.
• No GPU execution or timing contract change. Exact-head Full CI is requested after this signed update.

Stack

• Draft-only stack on the B200 campaign branch. Merge remains gated on review, exact-head Full CI, and a suitable main-targeted integration; no default promotion is included.

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[simone-chen]

/ok to test 4b73dc1

[simone-chen]

/ok to test 6ca136f

[simone-chen]

Closing as outside the requested review scope: retain only attention #604, recurrent/MoE #605, and communication #591 for Tianhao's review. This campaign infrastructure change is not merged or included in the replacements. Its branch and history remain available for separate future work; collection remains stopped.