[iamparmjeet]

Implements remediation slice B06 (docs/Fix-Plan-2026-09-05.md). Terra High review owed before [x].

Scope: one reversal per credit + atomic reversal-insert/original-update on Node and Neon paths. One migration (0028, partial unique index), no data rewrite.

Changes:

• reverseCredit: idempotent pre-check returns existing pair; uniform insert, catch 23505, adopt winner, complete original (abort-safe on tx path); insert-first order kept
• extracted markCreditReversed + findReversalByCredit + wrap-aware violationCode
• credit-reversal.test.ts: 5 tests (written first)

Behavior change: retry returns the pair instead of CONFLICT Already reversed (nothing depended on it).

Verification: db:generate no drift, check-types --force 6/6, Biome clean, db:migrate:test, full suite 236/236, local build 5/5. Dev has zero credit reversals.

Rollback: tag pre-atomic-credit-reversal on integ/phase-a-baseline@be7aca91.

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