dash: S8 embedded P2P relay arm — won-block dual-path broadcaster

[frstrtr]

S8 — DASH won-block dual-path broadcaster (embedded P2P relay wired)

Closes the DASH broadcaster dual-path gate. DASH was single-path: a won block broadcast only via the submitblock-RPC arm (external dashd, proven live at block 2507753). The embedded P2P relay code existed but was unwired into main_dash's won-block sink, so DASH could not broadcast daemonless. This wires the embedded arm — mirroring the DGB reference that already passes this gate (make_on_block_found / block_broadcast.hpp).

What changed (fenced to src/impl/dash + main_dash)

• src/impl/dash/coin/won_block_dispatch.hpp (new) — dash::coin::broadcast_won_block() fires a won block down BOTH arms, a contract-mirror of dgb::coin::broadcast_won_block conformed to the DASH stratum path (block already reconstructed to wire bytes by DASHWorkSource):
  • ARM A — embedded P2P relay (ALWAYS-PRIMARY, daemonless): the E1 CoinClient::submit_block_p2p_raw pushes the packed block onto the coin P2P net. With no local dashd, a won block reaches the network on this arm alone.
  • ARM B — submitblock RPC backup (on-demand): NodeRPC::submit_block_hex, fired whenever a local dashd is armed (also covers a cold/faulted relay; ignore_failure=true so a duplicate after an ARM-A accept never masks the win).
  • Each arm is try/catch-guarded so a throwing sink can never skip the other; reaching NEITHER sink logs LOUDLY and reports any()==false — never a silent lost subsidy.
• src/c2pool/main_dash.cpp — run_node builds the P2pRelaySink (posts the CoinClient submit onto the io thread; empty when no --coin-p2p-connect peer or --no-p2p-relay) and the RpcSubmitSink (empty when no dashd creds), and routes the DASHWorkSource won-block sink through broadcast_won_block. New --no-p2p-relay flag suppresses ARM A only (A/B isolation, matching DGB's toggle). Run-loop banner now reports both arms' armed state. Stale "block-submission deferred / S8" comments updated.
• test/test_dash_won_block_dualpath.cpp (new) — forced-won-block KAT mirroring dgb_forced_won_share_dualpath_test. 5 cases: both arms carry byte-identical bytes/hex + cross-arm identity; daemonless P2P-only reaches network; RPC-only reaches network; no-arm reaches NEITHER sink (fail-loud); throwing-primary falls through to backup. Registered in test/CMakeLists.txt.

Posture

Embedded P2P relay = always-primary (daemonless); submitblock-RPC = on-demand backup (fires with local dashd OR relay fault). Never silent-drop. Reward/consensus-neutral — broadcast path only (no PoW/share/coinbase-commitment/PPLNS touched). Per-coin isolation held: other coins byte-unchanged.

Local verification (this branch)

• conan install + cmake configure: OK
• cmake --build --target c2pool-dash: builds + links clean
• cmake --build --target test_dash_won_block_dualpath + run: 5/5 PASS
• c2pool-dash --run --testnet --no-p2p-relay --stratum 13333: banner prints ARM A ... = SUPPRESSED (--no-p2p-relay) / ARM B ... = off (no dashd creds) and stands up/tears down cleanly.
• tools/ci/check_test_target_allowlist.py: OK (116 targets).

⚠️ ACTION FOR MERGE (workflow-scope): one build.yml line

The new KAT must be added to the build.yml --target allowlist (both CI legs, lines ~141 and ~323) or it becomes a NOT_BUILT/CTest-Not-Run sentinel. This repo's OAuth token lacks workflow scope, so the branch could not carry the .github/workflows/build.yml edit (push + Git Data API both denied on the workflow path). Please apply this at the two-party merge (append the token to the DASH group in BOTH --target lists):

... test_dash_get_work test_dash_stratum_work_source test_dash_won_block_dualpath \

Draft — integrator reviews + two-party merge (no self-merge). CI's dual-path KAT leg goes green once the build.yml token above is applied (verified locally: builds + 5/5).