Five tests pin down the contract of `migrate_legacy_stage_strings`:
rewrite of all pre-934 directory-style strings to clean wire form,
the lossy `7_frozen` → backlog + frozen-flag collapse, no-op on
already-clean items, idempotence, and graceful behaviour before
CRDT init. A test-only `seed_with_raw_stage` helper bypasses the
boundary normalisers (which can't produce legacy strings) by writing
directly to the CRDT register — the same shape we'll see in real
pre-migration data.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Timmy
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