M3 — wire external-daemon RPC transport into the build

Closes the deferred Option-B wiring from #149. The M3 RPC transport (rpc.cpp + the rpc_request.hpp/softfork_check.hpp SSOTs) landed as additive coin-layer source in #149 but was never registered in CMake, so it never compiled into the build — and the node owner still constructed the 1-arg stub NodeRPC.

Changes (src/impl/dgb/ only)

• coin/CMakeLists.txt: add rpc.cpp + rpc_request.hpp + softfork_check.hpp to dgb_coin_impl so the transport TU compiles and links.
• coin/node.hpp: capture the io_context in the Node ctor and construct the real NodeRPC(m_context, this, m_config->m_testnet), replacing the stub. This is the external-daemon FALLBACK path V36 mandates persists alongside the embedded daemon; testnet drives NodeRPC::check() chain-identity genesis probe.

Verification (local)

• dgb_coin + c2pool-dgb build EXIT=0 (rpc.cpp now compiles into libdgb_coin.a, exe links).
• M3 guard tests 30/30: rpc_request 8, softfork 8, genesis 7, dgb_share 5, subsidy 2.

CI notes

• No new test target → no build.yml allowlist change needed; rpc.cpp compiles via the existing dgb smoke (builds c2pool-dgb → links dgb_coin).
• Touches the live transport TU → ASan+UBSan gate is the acceptance check (as integrator flagged for the transport path).

p2pool-dgb-scrypt parity preserved. HOLD merge for operator push-approval.