Verify before signing, and sign on an offline machine

The online machine has addresses only. The offline machine has the keys.

1. Online: recipes/recover.sh FILE - --goal balance > utxos.jsonl, then tx-build --chain btc --utxos utxos.jsonl --to DEST --sweep --fee-rate 5 > unsigned.json.

2. Copy unsigned.json to the offline machine (USB). Check it there:

   ./blocktools run 'tx-verify --expect-to 1Q2TWHE3GMdB6BZKafqwxXtWAWgFt5Jvm3 --max-fee 10000 < /in/block170.hex'

   (Here the check correctly fails: a raw transaction has no input amounts, so the fee is unknown. A PSBT from tx-build carries them, so the check can pass.)

3. Offline, a human signs: ./blocktools reveal 'seed-repair < /in/seed.txt | seed-to-master | derive --chain btc | tx-sign --tx /in/unsigned.json --expect-to DEST' with a typed code. Then copy the signed tx back.

4. Online: tx-decode --text < signed.json, then tx-broadcast --chain btc < signed.json (a second code).