Why: X3 must share one Backend surface with the X1 context and X2 audit candidates while current main now owns account migrations 32-34.
Evidence: go test -p 1 ./... and go build ./... passed on the merged tree; isolated PostgreSQL project audit, migration, and work-context tests passed against a temporary instance. Default parallel go test ./... had only the unrelated sandbox orphan timing test fail; its isolated ten-run replay passed.
Impact: Preserve account migrations 32-34, place X1/X2 migrations at 35-43, and retain account-effective write guards alongside project audit/context behavior. X3 observer remains a contract only.
Refs: TT-C01-TT-C04, TT-A01-TT-A04, TT-N03, TT-R01; combination probe d9b00c2
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