Release-blocking bug: won DASH blocks rejected bad-cb-payee

c2pool-dash could build a coinbase that dashd rejects with bad-cb-payee when a block is found — the won block (and its reward) was silently lost. This gates the DASH release.

Hex-confirmed root cause (live testnet dashd, GBT proposal mode, 2026-07-19)

compute_dash_payouts silently dropped any GBT-mandated payment (masternode / superblock / platform burn) whose payee string failed decode_payee_script (if (pm_script.empty()) continue;), and normalize_payment discarded dashd's GBT-provided raw "script" scriptPubKey bytes whenever a non-empty base58 payee was present — so there was no byte-faithful fallback. The coinbase then lacked a consensus-REQUIRED output and dashd rejected every won block with bad-cb-payee.

Deterministic field trigger: net-mode/daemon-chain mismatch. A mainnet-mode binary against a testnet dashd applies address_version=76 to the y... testnet masternode payee → base58 decode fails → the masternode output vanishes. NodeRPC::check() passed this mismatch silently (it only probed the mainnet-genesis direction).

Evidence (same daemon, same height-1517409 template, byte diff):

• testnet mode: coinbase = worker ‖ platform-burn 6a (66966830) ‖ MN payee 76a91464f2b2b84f62d68a2cd7f7f5fb2b5aa75ef716d788ac (111611384) ‖ donation ‖ OP_RETURN ‖ CbTx → dashd proposal verdict null (valid)
• mainnet mode: identical except the MN payee output is MISSING (amount folded into donation) → dashd proposal verdict bad-cb-payee

Falsified alternates (per the parallel investigation): the fresh-template stratum coinbase on the right net is dashd-valid (proposal-accepted twice + a live won block ACCEPTED, below); template+coinbase+txs are frozen as ONE atomic DashWorkData snapshot per job, so intra-height GBT drift cannot desync payee vs body; tip-move staleness produces prev-based rejects (observed live: bad-chainlock), never bad-cb-payee.

Fix (fenced to src/impl/dash/ + tests)

1. coin/rpc_data.hpp — PackedPayment gains a script field; normalize_payment ALWAYS preserves the raw GBT scriptPubKey bytes alongside the payee string (payee-string semantics unchanged → share-wire compatible; the sharechain PackedPayment is a distinct struct).
2. coinbase_builder.hpp — compute_dash_payouts never drops: (a) decode the payee string as before; (b) on failure, emit the GBT script bytes verbatim with a LOUD warning; (c) if neither is usable, throw — the stratum path serves no job and screams, instead of letting a miner burn hashrate on a block dashd must reject.
3. coin/rpc.cpp — chain-identity guard in check(): mainnet mode requires chain=="main"; --testnet rejects chain=="main" (accepts test/regtest/devnet for the tuned-net harness posture). Wrong-net now fails loudly at connect.
4. coin/embedded_gbt.hpp — embedded arm also carries raw scriptPayout bytes for the same fallback.

KATs (existing allowlisted targets — no new test target, no build.yml change)

test_dash_cb_payee (12/12): raw-script preservation; right-net coinbase contains burn + MN payee with exact GBT script+amount (real captured testnet GBT entries); wrong-net falls back to GBT script verbatim (the pre-fix vanishing-output case); undecodable-without-script throws; GOLDEN — byte-for-byte full coinbase for the real height-1517409 template, the exact serialization dashd accepted live via proposal mode.
test_dash_coinbase_parity (11/11): updated to the never-drop rule + new UndecodablePayeeThrowsNeverDrops.

Verification

• test_dash_cb_payee 12/12, test_dash_coinbase_parity 11/11, test_dash_stratum_work_source 19/19, g3_assembled 9/9, block_producer 9/9, embedded_gbt 9/9, coinbase_muldiv, donation_combined, get_work, work_source, node_coin_state, rpc_request all green; --selftest OK.
• Live testnet (192.168.86.52): fixed binary's stratum-served coinbase accepted by dashd proposal mode (verdict null).
• Wrong-net now refuses at connect: Chain mismatch: dashd reports chain='test' but c2pool-dash is in MAINNET mode...
• Reward-safe gate PROVEN LIVE: fixed builder X11-mined and submitted a real testnet block — submitblock ACCEPTED, tip advanced to height 1517418 with our hash 0000007a8b36efe807cbc0ea37172d3267b93978a375c1c247045bb009efaa5b; on-chain coinbase carries the platform burn (0.66966830), the masternode payee yVXDAM73Tg6A44Bm3qduXsMCYxzuqBCT48 (1.11611384), donation, OP_RETURN ref slot, and the full CbTx payload. (Two prior wins during the run were rejected bad-chainlock — natural stale-tip races while CPU-mining, not payee failures.)