dgb: §4c AuxPoW comment tightening + aux seam signature

* dgb: tighten coin/node skeleton AuxPoW comment to V36 reality + aux seam signature

§4c skeleton extension (Slice #3). Documents the AuxPoW/merged-mining
posture on the dgb::coin::Node minimal-stub: DGB-Scrypt is a standalone
Scrypt-only parent in the default build (no AuxPoW path); merged-mining
is the DOGE-only stretch, gated behind -DAUX_DOGE=ON and served by the
shared src/impl/doge aux module (consumed, not owned). Adds the
bind_aux_doge_parsers() declaration only (no body) under #ifdef AUX_DOGE.

SAFE-ADDITIVE: comment + #ifdef-gated member declaration, never odr-used
in the default build. Single-coin src/impl/dgb only. No share_check
version-acceptance path, no stratum, no entrypoint. p2pool-merged-v36
parity preserved.

* dgb: §4c aux seam header-only type alias (slice #3, option b)

Add a header-only AuxChainBackend type alias plus a forward declaration of
doge::coin::AuxChainEmbedded under #ifdef AUX_DOGE, naming the shared
src/impl/doge aux-module backend that bind_aux_doge_parsers() binds onto the
M3 coin P2P layer. No new translation unit; the default Scrypt-only standalone
build never defines AUX_DOGE and is unaffected. dgb consumes the doge aux
module; it does not own or modify it. Parity: p2pool-merged-v36.

---------

Co-authored-by: frstrtr <frstrtr@users.noreply.github.com>

Author: frstrtr

src/impl/dgb/coin/node.hpp

@@ -13,19 +13,50 @@
 // The ctor keeps btc's (context, config) shape via auto params (no boost
 // include needed in the stub) so M3 restores the body without re-typing
 // construction sites.
+//
+// AuxPoW / merged-mining seam (V36 reality): DGB-Scrypt is a STANDALONE parent in
+// the default build and validates Scrypt blocks ONLY -- there is no AuxPoW path in
+// the default c2pool-dgb. Merged-mining is the DOGE-only stretch, compiled solely
+// under -DAUX_DOGE=ON, where the shared src/impl/doge/coin/aux_* module (owned by
+// ltc-doge; dgb CONSUMES, never modifies) supplies the AuxPoW header / raw-block
+// parsers that the M3 NodeP2P will bind. The other four DGB algos (SHA256d, Skein,
+// Qubit, Odocrypt) are accept-by-continuity / ignored in V36 -- never validated
+// here. p2pool-merged-v36 parity preserved. Signature only below; body lands with
+// NodeP2P at M3.
 // ---------------------------------------------------------------------------
 
 #include <memory>
 
 #include "node_interface.hpp"
 #include "rpc.hpp"
 
+#ifdef AUX_DOGE
+// DOGE merged-mining aux module (STRETCH; -DAUX_DOGE=ON only). dgb CONSUMES the
+// shared aux types; ltc-doge owns and is the sole modifier of src/impl/doge/coin/aux_*.
+// Forward declaration only at slice #3 -- the real
+// <impl/doge/coin/aux_chain_embedded.hpp> include lands with the
+// bind_aux_doge_parsers() body at M3.
+namespace doge { namespace coin { class AuxChainEmbedded; } }
+#endif
+
 namespace dgb
 {
 
 namespace coin
 {
 
+#ifdef AUX_DOGE
+// ---------------------------------------------------------------------------
+// DOGE merged-mining aux seam -- header-only type alias (slice #3, option b per
+// integrator UID-904). Names the shared src/impl/doge aux-module backend that
+// bind_aux_doge_parsers() wires onto the (M3) coin P2P layer. No new TU; no
+// effect on the default Scrypt-only standalone build (which never defines
+// AUX_DOGE). The structured parsers consumed at M3 bind time are the free
+// functions doge::coin::parse_doge_header / parse_doge_headers_message /
+// parse_doge_block (auxpow_header.hpp). Parity: p2pool-merged-v36.
+using AuxChainBackend = ::doge::coin::AuxChainEmbedded;   // IAuxChainBackend impl
+#endif
+
 template <typename ConfigType>
 class Node : public dgb::interfaces::Node
 {
@@ -48,6 +79,13 @@ class Node : public dgb::interfaces::Node
     }
 
     NodeRPC* rpc() { return m_rpc.get(); }
+
+#ifdef AUX_DOGE
+    // DOGE merged-mining aux seam (STRETCH; -DAUX_DOGE=ON only). Binds the shared
+    // src/impl/doge aux module's parsers onto the (M3) coin P2P layer. Declaration
+    // only -- no body until NodeP2P is ported. Parity: p2pool-merged-v36.
+    void bind_aux_doge_parsers();
+#endif
 };
 
 } // namespace coin