Add --min-rate to alw swap now

[LandynDev]

alw swap now reserves the miner's current quote and sends at once. A miner that re-quotes between the preview and the bid gets its lower rate pinned, and the deposit still goes out.

• New optional --min-rate <float>: directional, what the taker receives per 1 sent (the rate the CLI prints).
• After the reservation is live and before any send (auto-send or manual print), the CLI re-reads the reservation on chain and derives its pinned rate with rate_display_from_fixed + directional_rate.
• Under the floor: Reservation rate X below --min-rate Y; not sending, exit 1.
• Read fails, not our reservation, or rate not derivable: Reservation unreadable; not sending: <why>, exit 1.
• Without the flag nothing changes: no extra read.

An aborted reservation costs only the reservation fee, already paid at the bid. It expires at reserved_until, and the miner is free again.

Branched from 183ae17 (the rev optimal-allways-miner pins) so the miner can pin this commit directly.

Tests: tests/test_swap_now_min_rate.py (decision helper + swap now CLI harness).