/refine: edit yourself, survive it

Repair (Module 12, L12.2). A truncating rewrite of a 23-byte tools.json leaves 12 bytes and bricks a direct write with JSONDecodeError. The same write to a sandbox copy leaves the live file untouched and stores one rollback entry. A gate edit is refused. An 8-byte crash leaves the live file unreadable on overwrite and readable at 23 bytes when the commit is temp-then-rename. In-memory only; no model.

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