Make joinstr-rust compatible with Python reference implementation

[kwsantiago]

Summary

• Pool event and message serde now matches Python wire format (BTC float denomination, string relay/transport, expanded credentials, optional version field, new_pool type)
• Fee/amount logic matches Python: output amount = denomination - fee_rate * 100, input range validation (denomination+500 to denomination+5000), fee bounds check (N100 to N10000)
• Input registration sends base64-encoded signed PSBT as {"psbt": "<base64>", "type": "input"} instead of custom InputDataSigned format
• Tests updated for new validation ranges