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/Makefile.am

@@ -325,6 +325,7 @@ libbitcoin_wallet_a_SOURCES = \
   wallet/asyncrpcoperation_shieldcoinbase.cpp \
   wallet/crypter.cpp \
   wallet/db.cpp \
+  wallet/mnemonic.cpp \
   zcash/Note.cpp \
   transaction_builder.cpp \
   wallet/rpcdump.cpp \
@@ -361,6 +362,23 @@ crypto_libbitcoin_crypto_a_SOURCES = \
   crypto/sha512.cpp \
   crypto/sha512.h
 
+# Vendored trezor-crypto BIP39 (mnemonic seed phrases). Kept self-contained so
+# the same 24 words are compatible with SilentDragonXLite (tiny-bip39 0.6.2).
+crypto_libbitcoin_crypto_a_SOURCES += \
+  crypto/bip39/bip39.c \
+  crypto/bip39/bip39.h \
+  crypto/bip39/bip39_english.h \
+  crypto/bip39/pbkdf2.c \
+  crypto/bip39/pbkdf2.h \
+  crypto/bip39/hmac.c \
+  crypto/bip39/hmac.h \
+  crypto/bip39/sha2.c \
+  crypto/bip39/sha2.h \
+  crypto/bip39/memzero.c \
+  crypto/bip39/memzero.h \
+  crypto/bip39/options.h \
+  crypto/bip39/rand.h
+
 if EXPERIMENTAL_ASM
   crypto_libbitcoin_crypto_a_SOURCES += crypto/sha256_sse4.cpp
 endif