dgb: M3 softfork-detection standalone regression guard

Pin collect_softfork_names() (softfork_check.hpp) across all three
getblockchaininfo softfork encodings (array-of-objects / array-of-strings /
object-keyed) plus accumulation + scalar no-op robustness. Guards the
softfork-gate parser NodeRPC uses to confirm segwit before trusting the
Scrypt GBT path, BEFORE the deferred post-#145 wiring CI-links rpc.cpp.

Header-only + nlohmann + gtest, no transport link -- standalone, does not
enter the dgb OBJECT lib.

Author: frstrtr

src/impl/dgb/test/softfork_check_test.cpp

@@ -0,0 +1,108 @@
+// ---------------------------------------------------------------------------
+// dgb M3 softfork-detection regression guard.
+//
+// Pins collect_softfork_names() (softfork_check.hpp), the parser NodeRPC uses
+// to decide whether "segwit" (and friends) are active on the external
+// digibyted BEFORE c2pool-dgb trusts the Scrypt getblocktemplate path. The
+// parser must tolerate all three getblockchaininfo["softforks"]/["bip9_softforks"]
+// encodings shipped by different DigiByte Core versions:
+//
+//   1. array of objects:  [{"id":"segwit",...}, ...]   (modern digibyted)
+//   2. array of strings:  ["segwit","taproot", ...]    (compact form)
+//   3. object with keys:  {"segwit":{...}, ...}         (BIP9 style)
+//
+// Header-only + nlohmann + gtest -- no boost::beast/jsonrpccxx transport, so it
+// builds standalone without entering the dgb OBJECT lib (same gate-safe shape
+// as rpc_request_test.cpp).
+// ---------------------------------------------------------------------------
+
+#include <gtest/gtest.h>
+
+#include <impl/dgb/coin/softfork_check.hpp>
+
+#include <set>
+#include <string>
+
+using dgb::coin::collect_softfork_names;
+using json = nlohmann::json;
+
+// --- Format 1: array of objects (modern digibyted) --------------------------
+
+TEST(DgbSoftforkCheck, ArrayOfObjectsCollectsIds)
+{
+    auto v = json::parse(R"([{"id":"segwit","active":true},{"id":"taproot"}])");
+    std::set<std::string> out;
+    collect_softfork_names(v, out);
+    EXPECT_EQ(out, (std::set<std::string>{"segwit", "taproot"}));
+}
+
+TEST(DgbSoftforkCheck, ArrayObjectMissingOrNonStringIdSkipped)
+{
+    // An object without a string "id" must be ignored, not crash the parse.
+    auto v = json::parse(R"([{"id":"segwit"},{"name":"noid"},{"id":42}])");
+    std::set<std::string> out;
+    collect_softfork_names(v, out);
+    EXPECT_EQ(out, (std::set<std::string>{"segwit"}));
+}
+
+// --- Format 2: array of strings (compact form) ------------------------------
+
+TEST(DgbSoftforkCheck, ArrayOfStringsCollectsValues)
+{
+    auto v = json::parse(R"(["segwit","taproot"])");
+    std::set<std::string> out;
+    collect_softfork_names(v, out);
+    EXPECT_EQ(out, (std::set<std::string>{"segwit", "taproot"}));
+}
+
+// --- Format 3: object keyed by softfork name (BIP9 style) -------------------
+
+TEST(DgbSoftforkCheck, ObjectCollectsKeys)
+{
+    auto v = json::parse(R"({"segwit":{"status":"active"},"taproot":{"status":"defined"}})");
+    std::set<std::string> out;
+    collect_softfork_names(v, out);
+    EXPECT_EQ(out, (std::set<std::string>{"segwit", "taproot"}));
+}
+
+// --- The contract that actually matters: segwit is detectable in every form -
+
+TEST(DgbSoftforkCheck, SegwitDetectedAcrossAllThreeEncodings)
+{
+    for (const auto& src : {R"([{"id":"segwit"}])", R"(["segwit"])", R"({"segwit":{}})"})
+    {
+        std::set<std::string> out;
+        collect_softfork_names(json::parse(src), out);
+        EXPECT_TRUE(out.count("segwit")) << "segwit not found in: " << src;
+    }
+}
+
+// --- Accumulation + robustness ---------------------------------------------
+
+TEST(DgbSoftforkCheck, AccumulatesIntoExistingSet)
+{
+    // Caller merges the "softforks" and "bip9_softforks" fields into one set;
+    // collect_softfork_names must append, never clear.
+    std::set<std::string> out{"preexisting"};
+    collect_softfork_names(json::parse(R"(["segwit"])"), out);
+    EXPECT_EQ(out, (std::set<std::string>{"preexisting", "segwit"}));
+}
+
+TEST(DgbSoftforkCheck, EmptyContainersYieldNothing)
+{
+    std::set<std::string> out;
+    collect_softfork_names(json::parse("[]"), out);
+    collect_softfork_names(json::parse("{}"), out);
+    EXPECT_TRUE(out.empty());
+}
+
+TEST(DgbSoftforkCheck, ScalarValueIsNoOp)
+{
+    // A missing field deserialized as null/number/string must be a safe no-op
+    // (the field may be absent on very old daemons).
+    std::set<std::string> out;
+    collect_softfork_names(json(nullptr), out);
+    collect_softfork_names(json(7), out);
+    collect_softfork_names(json("segwit"), out);  // bare scalar, not a container
+    EXPECT_TRUE(out.empty());
+}