fix(storage): preserve divergent session variants

[ScriptedAlchemy]

Summary

• preserve divergent session projections and raw content as deterministic active variants
• remap dependent references while keeping equal-content raw representation target-canonical
• migrate safe pre-merge consolidation ledgers to algorithm v2 and fail closed post-merge

Verification

• cargo test --lib migrate::consolidate::tests
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo fmt --all -- --check
• git diff --check

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[ScriptedAlchemy]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Skip payload rows when their raw row is deduped

When a source raw message collides with the target on provider/message_id but only differs in representation (same content_hash, e.g. source external payload_ref B while the target is inline or uses payload_ref A), raw_content_divergent is 0 so the raw row is intentionally not inserted and consolidation_raw_map points at the target raw row. This block still inserts the source lcm_external_payloads row whenever B is absent from target_input, leaving a payload owner/file in the destination that no lcm_raw_messages row references; the verifier mirrors this union, so it will not catch the orphan. Gate these payload rows on the source raw row actually being preserved/remapped rather than on payload_ref absence alone.

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