fix(compaction): scope legacy checkpoint recognition by era

[asto18089]

What

The restore lane already prefers the provenance-stamped compaction carrier, but the recompaction family still ran the loose predicate, which matches any user turn whose text merely starts with a legacy summary header. In a history that carries the stamped carrier, that misread silently drops the turn: last-round anchoring walks past it, old-turn retention skips it, and the coverage floor classifies user prompts through the same loose predicate, so it cannot catch the loss. The survival contract meanwhile counts the retained turn as a duplicated summary, so a replacement that does keep the turn fails closed — fixing the drop alone would have made every such rewrite refuse.

Checkpoint recognition in the recompaction family is now era-scoped through history_is_post_provenance:

• A history that contains a stamped carrier recognizes only stamped carriers as checkpoints. A pasted bare-header turn anchors the round, is retained verbatim, and does not count as a duplicated summary.
• A history without a stamped carrier predates the stamp — there the pasted header is indistinguishable from the saved bare carrier, so the loose form keeps applying, now pinned explicitly.
• The receipt count reads the replacement in its own era, except that a post-provenance original raises the bar to a stamped carrier: the engine stamps every replacement it builds, and the stamp is the only form request rewriting relocates, so a stampless replacement of a modern history is a dropped receipt even where a retained pasted turn would loosely count as one.
• Retention derives the era from the history the slice belongs to, not the slice: the pre-round prefix's carrier may sit inside the round.
• The /context inspector's last-round heuristic reads the same era-aware anchoring.

validate_survival_contract.mjs mirrors the era rule and the fixture matrix gains the pasted-header pass/fail pair; SURVIVAL_CONTRACT.md documents the raised receipt bar and the era signal (the history's content, not the session's age — one-way and destructible: /edit or a host rewind can truncate away the only stamped carrier).

Tests

• pasted_legacy_header_turn_is_content_in_a_post_provenance_history — the pasted turn anchors the round, is retained verbatim, a replacement keeping it validates with exactly one receipt, and dropping it fails the coverage floor.
• legacy_history_still_recognizes_a_bare_header_checkpoint — the legacy-era boundary both ways.
• Two fixture-matrix cases (pasted-legacy-header-survives-post-provenance pass / -dropped- fail) run through both the Rust enforcement and the mjs validator (16 fixtures green).
• cargo test -p codewhale-tui --lib compaction 186 passed / 0 failed; clippy -D warnings clean (lib).

Ported from the Pinvou/CodeWhale fork (#103), rescoped to the recompaction family: upstream's wire-only edit targeting and restore preference are already stricter than the era result there, so those lanes are untouched.

No-Issue: recompaction dropped a pasted legacy-header user turn in a history with a stamped checkpoint (port of fork PR #103)

[claude]

Claude Code Review

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[Hmbown]

🤖 This reply is from Claude Opus 5.5, an AI agent working on behalf of the Codewhale maintainers. It is posted through @Hmbown's account, but Hunter did not write it.

Merged in 24077dc. Thank you @asto18089. The era rule closes a real gap: in a history that already has a stamped carrier, a pasted Codex handoff could no longer anchor the round, and the duplicate-summary count treated it as a second summary. Your matrix cases show both.

One doc follow-up on main (e3e5632). A session that has never been compacted has no stamped carrier either, so a pasted legacy header still reads as a checkpoint at the first compaction. SURVIVAL_CONTRACT.md and the history_is_post_provenance doc now say that, instead of "predates stamping". Not in a release yet.

— Written by Claude Opus 5.5 (AI agent) on behalf of the maintainers.