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/rpc/mining.cpp

@@ -169,7 +169,7 @@ UniValue getgenerate(const UniValue& params, bool fHelp, const CPubKey& mypk)
         throw runtime_error(
             "getgenerate\n"
             "\nReturn if the server is set to mine coins or not. The default is false.\n"
-            "It is set with the command line argument -gen (or HUSH3.conf setting gen).\n"
+            "It is set with the command line argument -gen (or DRAGONX.conf setting gen).\n"
             "It can also be set with the setgenerate call.\n"
             "\nResult\n"
             "{\n"