dgb: wire -DAUX_DOGE build-flag seam (phase DA) (#310)

Un-stub the parked AUX_DOGE option in the dgb module CMakeLists so the
DGB-as-DOGE-parent dual-parent merged-mining stretch can be opted into at
configure time. When -DAUX_DOGE=ON the AUX_DOGE compile macro is defined
across the dgb subtree, compiling the inert aux seam already present in
coin/node.hpp (forward-decl of doge::coin::AuxChainEmbedded, the
AuxChainBackend type alias, and the bind_aux_doge_parsers() declaration --
no body, never ODR-used). Default OFF leaves the standalone Scrypt-only
parent build byte-unchanged with zero DOGE coupling.

The shared src/impl/doge aux module (ltc-doge owns) is still only CONSUMED
at M3/DB, when the real aux_chain_embedded.hpp include and the bind body
land; this slice adds no shared-module dependency. Prereq #82 (own-block
broadcaster) is closed.

Verified: configure -DCOIN_DGB=ON -DAUX_DOGE=ON EXIT=0 (option message
fires); dgb_aux_parent_coinbase_parity_test builds EXIT=0 under the macro
and runs 4/4 green.

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

Author: frstrtr

src/impl/dgb/CMakeLists.txt

@@ -6,9 +6,10 @@
 # Other 4 DGB algos (SHA256d, Skein, Qubit, Odocrypt) = accept-by-continuity
 # or ignored. Full 5-algo validation is DEFERRED TO V37.
 #
-# -DAUX_DOGE opt-in (DGB-as-DOGE-parent, the dual-parent stretch) is NOT wired
-# in this module yet: parked behind the Phase 5.8 settle (ltc-doge owns the
-# shared DOGE-aux surface). Do not add AUX_DOGE plumbing here at M2.
+# -DAUX_DOGE opt-in (DGB-as-DOGE-parent, the dual-parent stretch): the build-flag
+# seam is wired below (phase DA, post-#82). It only compiles the INERT aux seam in
+# coin/node.hpp; the shared DOGE-aux surface (ltc-doge owns) is CONSUMED at M3/DB,
+# not here. Default OFF -> standalone Scrypt-only parent.
 #
 # LIB GATING (mirrors btc/ltc): the dgb_coin (coin-layer), dgb_stratum
 # (miner-facing IWorkSource) and dgb (pool-layer) OBJECT libs are defined
@@ -17,6 +18,20 @@
 # of the flag (gating them behind COIN_DGB makes the linker fall back to raw
 # -ldgb/-ldgb_coin/-ldgb_stratum and fail). Only the test subtree stays behind
 # COIN_DGB; build.yml sets -DCOIN_DGB=ON and builds those targets.
+# -DAUX_DOGE=ON: DGB-as-DOGE-parent dual-parent merged-mining STRETCH opt-in
+# (phase DA). Defines the AUX_DOGE compile macro across the dgb subtree so the
+# inert aux seam in coin/node.hpp compiles (forward-decl + AuxChainBackend type
+# alias + bind_aux_doge_parsers() declaration -- no body, never ODR-used). The
+# shared src/impl/doge aux module is CONSUMED only at M3/DB, when the real
+# <impl/doge/coin/aux_chain_embedded.hpp> include and the bind body land
+# (ltc-doge owns and is the sole modifier of that surface). Default OFF keeps the
+# standalone Scrypt-only parent build with zero DOGE coupling.
+option(AUX_DOGE "DGB-as-DOGE-parent dual-parent merged-mining stretch (consumes shared src/impl/doge aux module at M3)" OFF)
+if(AUX_DOGE)
+  message(STATUS "c2pool-dgb: AUX_DOGE dual-parent stretch ENABLED (DGB-as-DOGE-parent)")
+  add_compile_definitions(AUX_DOGE)
+endif()
+
 add_subdirectory(coin)
 add_subdirectory(stratum)   # dgb_stratum: IWorkSource (#82 miner-facing path)
 # add_subdirectory(daemon)