fix(net): enforce nMinimumChainWork in IBD (eclipse / low-work fake-chain protection)

nMinimumChainWork was defined in chainparams but never checked, and IsInitialBlockDownload
decided "synced" from tip timestamp/height alone -- so an eclipsed or bootstrapping node
could be fed a cheap low-work fake chain with recent timestamps and trust it. Reset the
stale mainnet floor (0x281b32ff3198a1 was ABOVE the live chain, would have bricked mainnet)
to the real chainwork at height ~3,100,000, and hold a node in IBD until its tip reaches the
floor. Gated to the DRAGONX symbol so ephemeral assetchains from the same binary are not
trapped in IBD; the check can only keep a node in IBD, never force it out (no false-sync risk).
Complements the header-flood fix (b9fdc7981): that stops invalid-PoW headers off the real tip;
this stops valid-but-cheap fake chains from a fake genesis.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main.cpp

@@ -2456,6 +2456,17 @@ bool IsInitialBlockDownload()
         //fprintf(stderr,"nullptr in IsInitialDownload\n");
         return true;
     }
+
+    // SECURITY: enforce the known-good minimum chain work (defined in chainparams but previously
+    // never checked). Keeps an eclipsed/bootstrapping node from trusting a cheap low-work fake
+    // chain -- a recent tip timestamp alone (below) is not sufficient. Gated to the DRAGONX symbol
+    // so ephemeral assetchains (fresh, low work) run from the same binary are not trapped in IBD.
+    if (strncmp(SMART_CHAIN_SYMBOL, "DRAGONX", 7) == 0 &&
+        ptr->chainPower.chainWork < UintToArith256(chainParams.GetConsensus().nMinimumChainWork))
+    {
+        return true;
+    }
+
     state = ((chainActive.Height() < ptr->GetHeight() - 24*60) ||
              ptr->GetBlockTime() < (GetTime() - nMaxTipAge));
     if ( HUSH_INSYNC != 0 )