ci: build the DGB module in CI (-DCOIN_DGB=ON) + add dgb_share_test target

Two coupled fixes so the Phase-B DGB stack is actually build-verified by CI:

1. Set -DCOIN_DGB=ON in the Linux and ASan configure steps. The dgb
   coin module (src/impl/dgb) is gated behind if(COIN_DGB); no option()
   defaults it ON, so CI never compiled the module. test_dgb_subsidy
   passed only because it lives in the global test/ dir, outside the
   gate. Enabling the flag compiles the dgb OBJECT lib + dgb_coin +
   pool/share TU for the first time under CI (Linux + ASan/UBSan).

2. Add dgb_share_test to the --target allowlist in both jobs. It is
   defined in src/impl/dgb/test (inside the COIN_DGB gate) and links the
   dgb OBJECT lib, so it only exists once (1) sets the flag. Mirrors the
   #137 NOT_BUILT lesson: a gtest target must be in the build allowlist.

Author: frstrtr

.github/workflows/build.yml

@@ -44,7 +44,7 @@ jobs:
         run: conan install . --build=missing --output-folder=build_ci --settings=build_type=Release
 
       - name: Configure
-        run: cmake -S . -B build_ci -DCMAKE_TOOLCHAIN_FILE=build_ci/conan_toolchain.cmake -DCMAKE_BUILD_TYPE=Release
+        run: cmake -S . -B build_ci -DCMAKE_TOOLCHAIN_FILE=build_ci/conan_toolchain.cmake -DCMAKE_BUILD_TYPE=Release -DCOIN_DGB=ON
 
       - name: Build
         run: cmake --build build_ci --target c2pool -j$(nproc)
@@ -67,7 +67,7 @@ jobs:
                     test_phase4_embedded \
                     test_mweb_builder \
                     test_address_resolution test_compute_share_target \
-                    test_utxo test_dgb_subsidy \
+                    test_utxo test_dgb_subsidy dgb_share_test \
                     v37_test \
             -j$(nproc)
 
@@ -171,6 +171,7 @@ jobs:
           cmake -S . -B build_asan \
             -DCMAKE_TOOLCHAIN_FILE=build_asan/conan_toolchain.cmake \
             -DCMAKE_BUILD_TYPE=Release \
+            -DCOIN_DGB=ON \
             -DCMAKE_CXX_FLAGS="-fsanitize=address,undefined -fno-sanitize=vptr -fno-omit-frame-pointer -fno-sanitize-recover=undefined -g" \
             -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=address,undefined -fno-sanitize=vptr" \
             -DCMAKE_SHARED_LINKER_FLAGS="-fsanitize=address,undefined -fno-sanitize=vptr"
@@ -193,7 +194,7 @@ jobs:
                     test_phase4_embedded \
                     test_mweb_builder \
                     test_address_resolution test_compute_share_target \
-                    test_utxo test_dgb_subsidy \
+                    test_utxo test_dgb_subsidy dgb_share_test \
                     test_coin_broadcaster test_multiaddress_pplns test_pplns_stress \
                     v37_test \
             -j$(nproc)