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/hush_seed_nodes.txt

@@ -1,27 +1,17 @@
-# This is a list of nodes which hushd attempts to connect to automatically
+# This is a list of nodes which dragonxd attempts to connect to automatically
 # at start up time. You can check to see if they are up/down with:
 # cd contrib; cat hush_seed_nodes.txt | ./hush_scanner
 
-# IP/tor/i2p seeds from src/chainparamsseeds.h
-185.241.61.43
-87.251.76.166
-45.82.68.233
-87.251.76.33
-137.74.4.198
-149.28.102.219
-155.138.228.68
-107.174.70.251
-# hush_scanner uses nc which cannot deal with these
-# iljqq7nnmw2ij2ezl334cerwwmgzmmbmoc3n4saditd2xhi3xohq.b32.i2p
-# [2a0c:b641:6f1:34::2]
-# [2a0c:b641:6f1:c::2]
+# IP seeds from src/chainparamsseeds.h
+212.56.41.63
+194.140.198.176
+212.56.41.47
+144.126.147.165
+176.126.87.241
 
 # Hostname Seeds from src/hush_utils.h
-node1.hush.is
-node2.hush.is
-node3.hush.is
-node4.hush.is
-node5.hush.is
-node6.hush.is
-node7.hush.is
-node8.hush.is
+node1.dragonx.is
+node2.dragonx.is
+node3.dragonx.is
+node4.dragonx.is
+node5.dragonx.is