dash(replay): early-era (pre-V20) non-rotated quorum members — resolve the h=1081595 LLMQ_50_60 fold poison

[frstrtr]

Draft. Do not merge. Stacked on #1276 (integrator/dashd-cut-header-backfill-canserve).

The poison (live-proven)

The daemonless full-DIP3 self-derive fold (dash-selfderive-hdrfix.log) folded 1028161..1081594 byte-clean (folded=53434, roots_matched=53434, payees_verified=34395, DIVERGED=none) — #1274 payee-DIP3 enforcement proven — then failed CLOSED at h=1081595 on the FIRST non-null early LLMQ_50_60 (llmqType=1) qfcommit:

fold FAILED at h=1081595 tx[1]: quorum-member resolver has no member set
for llmqType=1 quorumHash=0000000000000001... — failing closed

quorum_lane_refusals=53435 (every block).

Root cause (NOT commitment_is_null)

commitment_is_null() is already dashd CFinalCommitment::IsNull-faithful and unchanged. The h=1081595 commitment is genuinely non-null (41/50 signers, real quorumPublicKey / quorumVvecHash / sigs) — mainnet's first mined LLMQ_50_60 DKG era (May 2019) with 9 invalid members that dashd's HandleQuorumCommitment PoSe-punishes. A member set is required for MN-list parity.

The gap is member production below the V20 floor. QuorumReplayEngine::observe_block() hard-refuses every block under v20_floor (mainnet 1'987'776) — the CL-modifier era, DIP-0024 rotation and the merkleRootQuorums self-check are all post-V20 / Phase-2 — so the W4 lane produced no member set the whole early-DIP3 run and the W1 fold's punish pass fell to the fail-closed site.

Fix — faithful dashd pre-V20 ComputeQuorumMembers (verified vs dashpay/dash develop)

For a non-rotated DKG base block B (B % dkgInterval == 0) below the floor:

• member list = GetListForBlock(B) — the list AS OF the base block, NO -8 work-block offset (the offset lives only in GetHashModifier's ChainLock lookup; ComputeQuorumMembers feeds the base-block list). Sourced as the W1 fold's just-proven DML at B (post_fold) = GetListForBlock(B) by construction.
• modifier = ::SerializeHash(make_pair(llmqType, blockHash(B))) — GetHashModifier's pre-V20 branch (DeploymentActiveAfter(pWorkBlockIndex, V20) is false), no ChainLock term. Byte-exact compute_quorum_modifier's CL-absent branch fed the BASE hash.
• selection = CalculateQuorum(size, modifier) over confirmed+valid MNs (compute_nonrotated_members / eligible() == dashd CalculateScores).

Keyed by (llmqType, blockHash(B)) via the same m_height_by_hash the forward path uses, so members_for(type, quorumHash) resolves a commitment whose quorumHash == its DKG base block hash. Iterates the chainparams llmq list (still carries LLMQ_50_60, mined in this era), not the runtime-enabled set. Strictly below the floor — at/above it observe_block() stays authoritative and is never shadowed. Wired into the bridge's post_fold hook (covers both the replay-fold and mn-diff-store drivers).

Reward-safe by construction: a wrong member set re-hashes to the wrong merkleRootMNList and the fold's per-block committed-cbTx-root self-check refuses forward — never a bad mint.

Tests (red→green)

test_dash_replay_quorum_seam:

• EarlyNonRotatedGateRefusesWithoutTheProducer — the poison gate: below the floor observe_block refuses and members_for is nullopt.
• EarlyNonRotatedProducerResolvesMembersFaithfully — GREEN: the producer resolves the LLMQ_50_60 set, byte-identical to dashd's pre-V20 ComputeQuorumMembers, banned MNs excluded, and the pre-V20 modifier is distinct from the post-V20 (work-block-hash) form.
• EarlyProducerIsANoOpAtOrAboveTheV20Floor — the below-floor scope.

test_dash_mn_state suite: 126 passed, 1 data-driven skip. Build BLS=REAL.

Live validation

Binary rebuilt; the self-derive run resumed from the existing capture (re-folds DIP3→1081595 from cached bodies). Acceptance: fold PAST h=1081595 with DIVERGED=none continuing and members resolving. (Full run to tip is ~days; not awaited here.)

[frstrtr]

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.