Add BIP39 seed phrases (SilentDragonXLite-compatible) and HD transparent keys

Derive transparent (t-addr) keys from the HD seed and add BIP39 mnemonic seed
phrases that are byte-for-byte compatible with SilentDragonXLite, so the same
24 words recover the same shielded and transparent addresses in either wallet.

HD transparent keys:
- Derive t-keys from the seed at m/44'/coin'/0'/0/i (were random CKeys).
- CHDChain gains a version-gated transparent counter; existing wallets load
  unchanged. GenerateNewKey routes through DeriveNewChildKey when enabled
  (-hdtransparent, default on).
- Restore from a seed hex via -hdseed with gap-limit pre-derivation; birthday
  pinned to genesis so the rescan is not clipped.

BIP39 seed phrases:
- Wire the vendored trezor BIP39 lib (src/crypto/bip39) into the build, fix its
  BIP39_WORDS guard, and disable the insecure mnemonic cache.
- Match SDXLite exactly: English wordlist, empty passphrase, PBKDF2 64-byte
  seed, coin type 141, ZIP-32 m/32'/141'/i' and BIP44 m/44'/141'/0'/0/i. Store
  the 32-byte entropy and expand to the 64-byte seed on demand.
- Restore via -mnemonic, create via -usemnemonic, reveal via z_exportmnemonic.

Verified by gtests including a known-answer BIP39 seed vector and z/t address
derivation checks (src/gtest/test_hdtransparent.cpp, test_mnemonic_compat.cpp).
Docs in doc/hd-transparent-keys.md and doc/seed-phrase.md.

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

src/rpc/server.h

@@ -353,6 +353,7 @@ extern UniValue nspv_listccmoduleunspent(const UniValue& params, bool fHelp, con
 extern UniValue getblocksubsidy(const UniValue& params, bool fHelp, const CPubKey& mypk);
 
 extern UniValue z_exportkey(const UniValue& params, bool fHelp, const CPubKey& mypk); // in rpcdump.cpp
+extern UniValue z_exportmnemonic(const UniValue& params, bool fHelp, const CPubKey& mypk); // in rpcdump.cpp
 extern UniValue z_importkey(const UniValue& params, bool fHelp, const CPubKey& mypk); // in rpcdump.cpp
 extern UniValue z_exportviewingkey(const UniValue& params, bool fHelp, const CPubKey& mypk); // in rpcdump.cpp
 extern UniValue z_importviewingkey(const UniValue& params, bool fHelp, const CPubKey& mypk); // in rpcdump.cpp