dash(replay-fold): gate payee cross-check on DIP3 ENFORCEMENT (fix h=1028163 self-derive divergence) [DRAFT]

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Port (B) — payee-ordering fold-correctness (DIP3-genesis regime)

Stacked on #1270 (dash/uaf-coldbridge-replayfold-store-exclusion). DRAFT, do not merge.

Root cause (NOT the tie-break)

The full-DIP3 self-derive replay poisoned at h=1028163 (3rd DIP3 block, 14 fresh MNs) with DML FOLD PAYEE MISMATCH even though merkleRootMNList MATCHED (computed==committed). The MN SET was right; the payee cross-check was wrong to run there.

I audited the tie-break the task fingered and it is already dashd-exact:

• payee_before() = memcmp(a.data(), b.data(), 32) < 0 == dashd uint256 operator< (base_blob::Compare, uint256.h:55/59) — the storage-byte order, not the base_uint arithmetic order that bit Tier-2 (dash(mempool): tx-serving block order — dashd GetHash() memcmp tie-break (ordering residual → 0) #1271).
• compare_by_last_paid_height / nRegisteredHeight / pass-5 bookkeeping all byte-match dashd CompareByLastPaid_GetHeight / RebuildListFromBlock.
• The SML-root match at the divergence height also proves the proTxHash byte layout is dashd-correct (it commits proRegTxHash).

The real bug: DIP3 has two heights. The deterministic list is built from ACTIVATION (1028160) but the coinbase paying GetMNPayee(list) is only ENFORCED from DIP0003EnforcementHeight (mainnet 1047200). dashd's own consensus rule CMNPaymentsProcessor::IsTransactionValid (masternode/payments.cpp:114) early-returns valid without checking the det payee when !DeploymentDIP0003Enforced(h) (deploymentstatus.h:53). In [activation, enforcement) the historical coinbase pays the legacy masternode winner, not the deterministic projection — so the cross-check is stricter than dashd there and false-poisons a byte-correct fold.

Fix (dashd-faithful, reward-safe)

Gate the payee cross-check on DeploymentDIP0003Enforced(H) (H >= dip0003_enforcement_height, default 1047200), mirroring dashd exactly.

• The merkleRootMNList SET self-check stays UNCONDITIONAL.
• The payee projection (pass 0) + nLastPaidHeight bookkeeping (pass 5) also stay unconditional — exactly as dashd BuildNewListFromBlock runs them from activation — so the payee axis is fully derived and self-consistent across the window and correct the instant enforcement begins.
• Production mines at tip (~2.5M), far above enforcement → money-path check UNCHANGED. No block is ever skipped; the fold now folds through the DIP3-genesis window instead of hard-stopping.

KAT (red → green), BLS=REAL

test_dash_replay_fold.cpp :: DashReplayFoldPreEnforcement — same state + same payment-block (coinbase pays nobody):

• AtEnforcementPayeeMismatchStillPoisons — H≥enforcement → poison (strict where dashd enforces).
• BelowEnforcementPayeeMismatchFoldsThrough — H<enforcement → folds through, SET self-check still binds, nLastPaidHeight still bumped.
• BelowEnforcementPayeePaidIsNotFlaggedSkipped — control.

Verified RED (forcing the old unconditional check poisons at h=102 with the exact merkleRootMNList self-check PASSED … PAYEE MISMATCH message — the h=1028163 signature) → GREEN (gated check folds through). Full test_dash_mn_state fold+prestate suites: 31/31 pass, no regressions.

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Superseded by #1293 (combined fetch+fold onion). This branch's head commit is present in #1293 by exact SHA (verified via triage). Closing to reconcile the draft backlog; the branch is preserved (not deleted) — reopen if #1293 changes course.