dgb: end-to-end GBT-parity KAT for embedded coinbasevalue

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What

End-to-end GBT-parity KAT for the embedded coinbasevalue. Drives a populated embedded Mempool through the production make_mempool_tx_source -> EmbeddedCoinNode -> build_work_template wiring (exactly src/c2pool/main_dgb.cpp:233) and asserts the emitted template coinbasevalue is bit-for-bit the figure digibyted getblocktemplate reports:

coinbasevalue == subsidy(next_height) + sum(included-tx fees)

across every DGB reward era (the captured p2pool-dgb-scrypt oracle vectors).

Why this slice

The existing tests pin each link in isolation:

• dgb_embedded_tx_select_test — make_mempool_tx_source shaping + fee sum
• dgb_embedded_coin_node_test — a stub tx source folds fees into coinbasevalue
• dgb_work_source_test — stratum path era subsidy + GBT-verbatim

None pinned the assembled chain equals GBT. Per integrator (2026-06-20), before the embedded subsidy+fees derivation may stand in for the external-daemon GBT coinbasevalue, parity must be KAT'd as EXACTLY equal.

GBT-parity verified: for every era boundary the populated-mempool embedded coinbasevalue equals oracle_subsidy + total_fees — the digibyted GBT figure — bit-for-bit. The KAT also pins:

• behaviour-preserving displacement: embedded (no-GBT) equals resolve_coinbase_value with the GBT figure present, when they agree;
• the HARD INVARIANT: a present GBT coinbasevalue is returned verbatim even on divergence (embedded never overrides a live digibyted figure).

Scope / safety

• Test-only, non-consensus — no production value change; fenced to src/impl/dgb/test/. Auto-mergeable per fenced-slice rules.
• Subsidy oracle = dgb::CoinParams::subsidy (bit-for-bit p2pool-dgb-scrypt replica).
• Links the full dgb_coin codec like dgb_embedded_tx_select_test; new target added to CMake + both build.yml --target allowlists (ci: build test_dgb_subsidy in both Linux test jobs (fix master-red NOT_BUILT) #143 NOT_BUILT trap).
• 4/4 PASS locally; GPG-signed.