The rescan_v2/v3/v4 chain has zero runtime consumers (nothing in core/,
generate/, teaching/, chat/ imports it). v2/v3 are frozen disk-snapshot
tests; v4 alone re-ran the LIVE reader and asserted a barrier-shift count +
admission set, so it broke on every legitimate reader improvement (it was 8
of the red tests). Its only real invariant (wrong=0) is guarded directly by
the serving runner and test_runtime_wrong_zero_preserved.
No ADR doc references the v4 layer (only S2/S3-post-rescan exist). Remove the
brittle live test plus the now-orphaned generator and its two artifacts.
Keep the frozen, tested v2/v3 layer as the historical record.
Net: -8 brittle tests, removes a maintenance-only archaeology surface rather
than greening it.
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