revert(net): remove header-accept RandomX check; keep nMinimumChainWork + #8

An adversarial re-review found the header-accept RandomX check (b9fdc7981 +
7e9b2c661 header-PoW + d2124a303 defer) to be a persistent source of
consensus-liveness bugs: it derives the RandomX key from the ACTIVE chain
(hush_chainactive), the wrong branch for reorg/side-branch/catch-up headers, so
it repeatedly false-rejected validly-mined headers and DoS(100)-hard-banned
honest peers (IBD-tail catch-up and deep-reorg cases); the defer fix and an
extend-tip fix each addressed one case while leaving/creating others (an
extend-tip variant re-opened an unbounded post-IBD side-branch flood). It only
mitigated a low-harm resource DoS -- forged headers bloat mapBlockIndex memory/
disk but are never SELECTED (nMinimumChainWork) and the full RandomX + target
check still runs at block-connect. Revert to fCheckPOW=0 at header-accept
(original behavior). A comment in AcceptBlockHeader records that any re-attempt
must derive the key from the header's OWN ancestry (pindexPrev->GetAncestor),
never the active chain.

Also hardens two issues the same review found:
- #8 IBD header cap now bounds against the VALIDATED chainActive.Height()
  (attacker-hard) instead of pindexBestHeader, which a forward-extending flood
  advanced in lockstep, defeating the cap.
- opreturn_burn only emits a change output above the dust threshold; a sub-dust
  change made the returned tx non-standard/unrelayable.

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

src/pow.cpp

@@ -804,20 +804,8 @@ bool CheckRandomXSolution(const CBlockHeader *pblock, int32_t height)
 
     // Derive the key (shared helper) and serialize the input (identical bytes to the pool path).
     std::string rxKey = GetRandomXKey(height);
-    if (rxKey.empty()) {
-        // The RandomX key block (keyHeight = ((height-lag)/interval)*interval, looked up on the
-        // ACTIVE chain) is not yet connected. This happens ONLY at header-accept when headers run
-        // ahead of our connected block tip (the IBD tail / catch-up) -- block-connect always has it,
-        // since blocks connect in order and keyHeight <= height-lag < the connected tip. The header
-        // is NOT invalid; we simply cannot verify it YET. Defer to block-connect (which re-checks
-        // with the key present) rather than returning an error -- returning an error here makes
-        // CheckBlockHeader DoS(100)-ban the honest peer that sent a perfectly valid tip header we
-        // just can't check yet (observed live: a post-reindex node banned the whole fleet and stalled
-        // ~2000 blocks short of the tip). Flood protection is preserved for synced nodes (key present
-        // -> real RandomX check) and bounded during catch-up by the per-peer IBD header cap + nMinimumChainWork.
-        LogPrint("net", "CheckRandomXSolution: RandomX key block for height %d not yet connected; deferring verification to block-connect\n", height);
-        return true;
-    }
+    if (rxKey.empty())
+        return error("CheckRandomXSolution(): cannot derive RandomX key for height %d", height);
     std::vector<unsigned char> ssInput = GetRandomXInput(*pblock);
 
     char computedHash[RANDOMX_HASH_SIZE];