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>

depends/packages/boost.mk

@@ -1,12 +1,10 @@
 package=boost
 
 $(package)_version=1_72_0
-#$(package)_download_path=https://boostorg.jfrog.io/artifactory/main/release/$(subst _,.,$($(package)_version))/source/
-#$(package)_file_name=$(package)_$($(package)_version).tar.bz2
+$(package)_download_path=https://archives.boost.io/release/$(subst _,.,$($(package)_version))/source
+$(package)_file_name=$(package)_$($(package)_version).tar.bz2
+$(package)_download_file=$(package)_$($(package)_version).tar.bz2
 $(package)_sha256_hash=59c9b274bc451cf91a9ba1dd2c7fdcaf5d60b1b3aa83f2c9fa143417cc660722
-$(package)_download_path=https://git.hush.is/attachments
-$(package)_file_name=7b13759e-8623-4e48-ae08-f78502f4b6a5
-$(package)_download_file=7b13759e-8623-4e48-ae08-f78502f4b6a5
 $(package)_patches=fix-Solaris.patch ignore_wnonnull_gcc_11.patch range_enums_clang_16.patch
 
 define $(package)_set_vars