Compliant rebrand: legal compliance, code audit, and documentation (Phases 0-3)

Phase 0 - Legal Compliance:
- COPYING: Add DragonX copyright, preserve all upstream attributions
- AUTHORS: Add DragonX developers section
- LICENSE: Restore standard FSF GPLv3 text (fix GNU→GENERAL error)
- Add DragonX copyright headers to all 21 modified source files
- contrib/debian/copyright: Add DragonX attribution
- README.md: Add GPLv3 Section 5(a) attribution section

Phase 1 - init.cpp Cleanup:
- PID file: hushd.pid → dragonxd.pid
- Shutdown thread: hush-shutoff → dragonx-shutoff
- Debug message: stopping HUSH → stopping DragonX

Phase 2 - HUSH3/ishush3 Code Audit:
- Rename ishush3 → isdragonx across ~15 source files
- Update "HUSH3" chain-identity checks to "DRAGONX" in consensus,
  difficulty, notarization, devtax, and RPC code
- Intentionally preserve cross-chain "HUSH3" refs (gateway, notary dest)
- Build verified: all three binaries compile cleanly

Phase 3 - Documentation:
- README.md: Full rewrite with DragonX chain params, build instructions
- Man pages: Create dragonxd.1, dragonx-cli.1, dragonx-tx.1 (v1.0.0)
- Doc files: Add beefy-DRAGONX.conf, dragonxd-systemd.md, dragonxd.service
- Init scripts: Create dragonxd.{conf,init,openrc,openrcconf,service}
- Debian packaging: Update control, changelog, install, manpages, examples
- Update doc/init.md and contrib/init/README.md

contrib/init/dragonxd.service

@@ -0,0 +1,22 @@
+[Unit]
+Description=Hush: Speak And Transact Freely
+After=network.target
+
+[Service]
+User=hush
+Group=hush
+
+Type=forking
+PIDFile=/var/lib/dragonxd/dragonxd.pid
+ExecStart=/usr/bin/dragonxd -daemon -pid=/var/lib/dragonxd/dragonxd.pid \
+-conf=/etc/hush/hush.conf -datadir=/var/lib/dragonxd -disablewallet
+
+Restart=always
+PrivateTmp=true
+TimeoutStopSec=60s
+TimeoutStartSec=2s
+StartLimitInterval=120s
+StartLimitBurst=5
+
+[Install]
+WantedBy=multi-user.target