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/chainparams.cpp

@@ -131,7 +131,9 @@ public:
         consensus.vUpgrades[Consensus::UPGRADE_SAPLING].nActivationHeight    = Consensus::NetworkUpgrade::NO_ACTIVATION_HEIGHT;
 
         // The best chain should have at least this much work.
-        consensus.nMinimumChainWork = uint256S("0x00000000000000000000000000000000000000000000000000281b32ff3198a1");
+        // DRAGONX mainnet chainwork @ height ~3,100,000 (2026-07), safely below the live tip.
+        // (Previous value 0x281b32ff3198a1 was a stale inherited figure ABOVE the live chain.) Bump on release.
+        consensus.nMinimumChainWork = uint256S("0x0000000000000000000000000000000000000000000000000026dfbdb6fa39e0");
 
         /**
          * The message start string is designed to be unlikely to occur in normal data.