Coin selection, change and the fee on the overlay came from the
Electrum server's listunspent values, and a legacy signature does not
commit to the value it spends. A server that under-reported a P2PKH
coin made Aegis sign away the difference as fee: a 1,000,000 sat coin
reported as 100,000 produced a transaction paying 901,180 sat while
the overlay said 1,180. Segwit v0 commits only to its own input, which
leaves the two-request variant open.
Every non-taproot input's previous transaction is now fetched, its txid
recomputed, and its output's value and script compared with the plan;
the fee of the finalized PSBT must equal the approved one.
