cleanup: rebrand residual user-facing "HUSH" strings to DragonX

Sweeps the leftover coin-name strings in RPC help text, RPC output, and log
messages that the currency-unit change didn't cover:

- RPC help: "mining reward amount in HUSH" -> DRAGONX (mining.cpp x2);
  "at least minbal HUSH" -> DRAGONX (rpcwallet.cpp); "the HUSH address" /
  "(string) HUSH address" -> DragonX (rawtransaction.cpp)
- RPC output: the SMART_CHAIN_SYMBOL[0]==0 ? "HUSH" : SYMBOL coin-name fallback
  (crosschain/misc/mining/blockchain) -> "DRAGONX"; the notarizations JSON key
  make_pair("HUSH", ...) -> "DRAGONX"
- Logs: "HUSH blocktime changing", "stopping HUSH HTTP/REST/RPC",
  "HUSH raw magic=" -> DragonX

Left untouched (verified): the 82 "HUSH3"/ishush3 chain-symbol consensus checks;
hush_globals.h CURRENCIES[] price-oracle basket (internal lookup, dead feature
on DragonX); hush.h notarization debug printf; a commented-out cout in main.cpp.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/rpc/blockchain.cpp

@@ -926,7 +926,7 @@ UniValue kvsearch(const UniValue& params, bool fHelp, const CPubKey& mypk)
     LOCK(cs_main);
     if ( (keylen= (int32_t)strlen(params[0].get_str().c_str())) > 0 )
     {
-        ret.push_back(Pair("coin",(char *)(SMART_CHAIN_SYMBOL[0] == 0 ? "HUSH" : SMART_CHAIN_SYMBOL)));
+        ret.push_back(Pair("coin",(char *)(SMART_CHAIN_SYMBOL[0] == 0 ? "DRAGONX" : SMART_CHAIN_SYMBOL)));
         ret.push_back(Pair("currentheight", (int64_t)chainActive.LastTip()->GetHeight()));
         ret.push_back(Pair("key",params[0].get_str()));
         ret.push_back(Pair("keylen",keylen));