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/hush_utils.h

@@ -1466,7 +1466,7 @@ uint32_t hush_smartmagic(char *symbol,uint64_t supply,uint8_t *extraptr,int32_t
     {
         vcalc_sha256(0,hash.bytes,extraptr,extralen);
         crc0 = hash.uints[0];
-        fprintf(stderr,"HUSH raw magic=");
+        fprintf(stderr,"DragonX raw magic=");
         int32_t i; for (i=0; i<extralen; i++)
             fprintf(stderr,"%02x",extraptr[i]);
         fprintf(stderr," extralen=%d crc0=%x\n",extralen,crc0);