Three improvements to WalletLock integration:
1. OpenExisting acquires the lock before decrypting the file — no point
attempting a potentially expensive PBKDF2 derivation for a wallet
already held by another instance.
2. OpenFromPath acquires the lock before CloseWallet — if the new wallet
is unavailable, the current session stays intact instead of being
silently destroyed.
3. OpenLoaded now receives an already-acquired WalletLock from the caller
instead of acquiring it internally; TryAcquireWalletLock is removed.
Callers that fail to open (wrong password, I/O error) dispose the lock
in their catch blocks.
Also fix CoinAmount.Of() to throw ArgumentException for unknown units
instead of silently falling back to PLM.
davide
58645bd7a7