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/wallet/rpcwallet.cpp

@@ -6437,8 +6437,11 @@ UniValue opreturn_burn(const UniValue& params, bool fHelp, const CPubKey& mypk)
     // Return the unspent surplus (selected inputs - burn amount - txfee) as change to a
     // wallet-owned address; without this the entire surplus is silently paid as miner fee.
     CAmount change = normalInputs - nAmount - txfee;
-    if ( change > 0 )
-        mtx.vout.push_back(CTxOut(change, GetScriptForDestination(myPubkey.GetID())));
+    // Only emit change if it clears the dust threshold; a sub-dust output would make the tx
+    // non-standard (unrelayable). Sub-dust surplus is folded into the fee (standard wallet behavior).
+    CTxOut changeOut(change, GetScriptForDestination(myPubkey.GetID()));
+    if ( change > 0 && !changeOut.IsDust(::minRelayTxFee) )
+        mtx.vout.push_back(changeOut);
     ret.push_back(Pair("hex", EncodeHexTx(mtx)));
 	return(ret);
 }