Compaction as a durable transaction

Build lab: an unchecked compactor commits a verbose 909-token summary of an 894-token transcript (ratio 1.02) and overwrites the original turns in place, while a naive any-shrink (< 1.00) compactor leaks a 536-token treadmill summary (ratio 0.60). Build a four-phase durable transaction (BEGIN -> SUMMARIZE -> VERIFY -> COMMIT | ROLLBACK) that enforces the 0.50 shrink invariant (< 447 tokens), fails closed, and archives the pre-image.

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