Rebrand hush3 to DragonX and share RandomX dataset across mining threads

Minimal rebrand (see compliant-rebrand branch for full rebrand):
- Rename binaries: hushd/hush-cli/hush-tx → dragonxd/dragonx-cli/dragonx-tx
- Default to DRAGONX chain params without -ac_* flags (randomx, blocktime=36, private=1)
- Update configure.ac: AC_INIT([DragonX],[1.0.0])
- Update client version string and user-agent to /DragonX:1.0.0/
- Add chainparams.cpp with DRAGONX network parameters
- Update build.sh, miner.cpp, pow.cpp for DragonX
- Add bootstrap-dragonx.sh utility script
- Update .gitignore for release directory

Share single RandomX dataset across all mining threads:
- Add RandomXDatasetManager with readers-writer lock, reducing RAM from
  ~2GB per thread to ~2GB total plus ~2MB per thread for the VM scratchpad
- Add LogProcessMemory() diagnostic helper for Linux and Windows

src/chainparamsbase.cpp

@@ -30,7 +30,7 @@ class CBaseMainParams : public CBaseChainParams
 public:
     CBaseMainParams()
     {
-        nRPCPort = 18031;
+        nRPCPort = 21769;
     }
 };
 static CBaseMainParams mainParams;