test: expand coverage to 209 tests across all core modules
Added tests for CoinAmount (all units, comma decimal, sub-satoshi rejection,
overflow, unknown unit ArgumentException, roundtrip), Storage (nonce/salt
uniqueness per encrypt, atomic write, WalletLock acquire/release/stale-file),
WalletLoader (NewFromMnemonic, passphrase isolation, watch-only, invalid
inputs, ProfileOf), SPV/Merkle (single tx, odd/even counts, altered proof,
empty list), and Chain (unknown NetKind, distinct profiles, port constants).
davide
ee0b73dd52