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/net.cpp

@@ -133,7 +133,7 @@ UniValue getpeerinfo(const UniValue& params, bool fHelp, const CPubKey& mypk)
             "    \"pingtime\": n,             (numeric) ping time\n"
             "    \"pingwait\": n,             (numeric) ping wait\n"
             "    \"version\": v,              (numeric) The peer version, such as 170002\n"
-            "    \"subver\": \"/GoldenSandtrout:x.y.z[-v]/\",  (string) The string version\n"
+            "    \"subver\": \"/DragonX:x.y.z[-v]/\",  (string) The string version\n"
             "    \"inbound\": true|false,     (boolean) Inbound (true) or Outbound (false)\n"
             "    \"startingheight\": n,       (numeric) The starting height (block) of the peer\n"
             "    \"banscore\": n,             (numeric) The ban score\n"
@@ -505,7 +505,7 @@ UniValue getnetworkinfo(const UniValue& params, bool fHelp, const CPubKey& mypk)
             "\nResult:\n"
             "{\n"
             "  \"version\": xxxxx,                      (numeric) the server version\n"
-            "  \"subversion\": \"/GoldenSandtrout:x.y.z[-v]/\",     (string) the server subversion string\n"
+            "  \"subversion\": \"/DragonX:x.y.z[-v]/\",     (string) the server subversion string\n"
             "  \"protocolversion\": xxxxx,              (numeric) the protocol version\n"
             "  \"localservices\": \"xxxxxxxxxxxxxxxx\", (string) the services we offer to the network\n"
             "  \"timeoffset\": xxxxx,                   (numeric) the time offset (deprecated, always 0)\n"