Two critical vulnerabilities allowed an attacker to flood the DragonX chain
with minimum-difficulty blocks starting at height 2879907:
1. ContextualCheckBlockHeader only validated nBits for HUSH3 mainnet
(gated behind `if (ishush3)`), never for HAC/smart chains. An attacker
could submit blocks claiming any difficulty and the node accepted them.
Add nBits validation for all non-HUSH3 smart chains, gated above
daaForkHeight (default 450000) to maintain consensus with early chain
history that was mined by a different binary.
2. The rebrand commit (85c8d7f7d) commented out the `return false` block
in CheckProofOfWork that rejects blocks whose hash does not meet the
claimed target. This made PoW validation a no-op — any hash passed.
Restore the rejection block and add RANDOMX_VALIDATION height-gated
logic so blocks after the activation height are always validated even
during initial block loading.
Vulnerability #1 was inherited from the upstream hush3 codebase.
Vulnerability #2 was introduced by the DragonX rebrand.
