cleanup: rebrand currency unit, depends mirrors, seeds; drop Hush-history files

Follow-up to the doc rebrand, addressing the previously out-of-scope legacy:

- Currency unit: strCurrencyUnits (chainparams.cpp) and CURRENCY_UNIT
  (amount.cpp) "HUSH" -> "DRAGONX". Both are display-only (RPC help + metrics);
  no logic comparisons, verified.
- depends mirrors: libsodium/boost/utfcpp fetched from git.hush.is/attachments;
  repointed to canonical upstream (GitHub release / archives.boost.io / GitHub
  tag) with the existing sha256 hashes verified to match those sources.
- Seeds: nodes_main.txt now lists the five node[1-5].dragonx.is IPs (DNS-resolved)
  instead of Hush nodes; regenerated src/chainparamsseeds.h (was compiling Hush
  seed IPs as the fixed fallback); generate-seeds.py header now says DragonX;
  hush_seed_nodes.txt updated to DragonX seeds.
- Deleted Hush-history / wrong-for-DragonX files: contrib/snapshot/ (block-500000
  Hush airdrop, ~10MB), notary_seeds.txt (Hush notaries; DragonX isn't notarized),
  and the Hush emission scripts hush_supply, hush_supply_old, hush_halvings,
  hush_block_subsidy_per_halving (hardcode Hush's 340000/12.5 economics).

Kept: hush_scanner (engine invoked by dragonx_scanner) and the "The Hush
developers" copyright headers (lineage credit).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

contrib/seeds/generate-seeds.py

@@ -167,9 +167,9 @@ def main():
     g.write('// Instead, update contrib/seeds/nodes_main.txt then run\n')
     g.write('//    ./contrib/seeds/generate-seeds.py contrib/seeds > src/chainparamsseeds.h\n')
     g.write('// OR run: make seeds\n')
-    g.write('#ifndef HUSH_CHAINPARAMSSEEDS_H\n')
-    g.write('#define HUSH_CHAINPARAMSSEEDS_H\n')
-    g.write('// List of fixed seed nodes for the Hush network\n')
+    g.write('#ifndef DRAGONX_CHAINPARAMSSEEDS_H\n')
+    g.write('#define DRAGONX_CHAINPARAMSSEEDS_H\n')
+    g.write('// List of fixed seed nodes for the DragonX network\n')
     g.write('// Each line contains a BIP155 serialized address.\n')
     g.write('//\n')
     with open(os.path.join(indir,'nodes_main.txt'), 'r', encoding="utf8") as f: