feat: wrap wallet events API (apply_update_events)

[darioAnongba]

Summary

Wrap BDK's WalletEvent type and add Wallet::apply_update_events so JS consumers can react to wallet state changes after syncing.

New API

WalletEvent type

JS-friendly wrapper with getters instead of Rust enum pattern matching:

Getter	Type	Description
kind	string	Event type: chain_tip_changed, tx_confirmed, tx_unconfirmed, tx_replaced, tx_dropped
txid	Txid?	Transaction id (tx events only)
tx	Transaction?	Full transaction (tx events only)
old_tip	BlockId?	Previous chain tip (chain_tip_changed only)
new_tip	BlockId?	New chain tip (chain_tip_changed only)
block_time	ConfirmationBlockTime?	Confirmation info (tx_confirmed only)
old_block_time	ConfirmationBlockTime?	Previous confirmation (reorgs)

Wallet.apply_update_events(update)

Like apply_update but returns WalletEvent[] describing what changed.

Example usage

const update = await esploraClient.full_scan(request, stopGap, parallelRequests);
const events = wallet.apply_update_events(update);
for (const event of events) {
  switch (event.kind) {
    case 'tx_confirmed':
      console.log(`TX ${event.txid} confirmed at block ${event.block_time.block_id.height}`);
      break;
    case 'chain_tip_changed':
      console.log(`Chain tip: ${event.old_tip.height} → ${event.new_tip.height}`);
      break;
  }
}

Closes #19