dgb(cmake): wire dgb_coin via add_subdirectory (Phase A #82)

Flip per OBJECT-lib convention: register src/impl/dgb at top level and enable the coin subdir under COIN_DGB. dgb_coin now builds standalone (-DCOIN_DGB=ON); libdgb_coin.a links clean, 0 errors. daemon/test stay commented (M3).

Author: frstrtr

src/impl/CMakeLists.txt

@@ -1,2 +1,3 @@
 add_subdirectory(ltc)
-add_subdirectory(btc)
+add_subdirectory(btc)
+add_subdirectory(dgb)

src/impl/dgb/CMakeLists.txt

@@ -18,7 +18,7 @@
 # follows the OBJECT-lib convention PR; held ~2-3 heartbeats, not raced here.
 if(COIN_DGB)
   message(STATUS "c2pool: DGB Scrypt-only coin module enabled (skeleton)")
-  # add_subdirectory(coin)
+  add_subdirectory(coin)
   # add_subdirectory(daemon)
   # add_subdirectory(test)
 endif()