Swaps: don't reuse a burned swap index (deterministic-preimage collision)

A reverse swap's preimage is sha256(swapNode(idx).privateKey) — deterministic per
index. nextSwapIndex was derived only from *saved* swaps, so an attempt that
created the swap at Boltz but threw before saving (e.g. the lockup-address-mismatch
bug) never advanced the index; the next try reused the same preimage and Boltz
rejected it ('a swap with this preimage hash exists already').

Make nextSwapIndex a persisted, monotonic counter (reserves the index on every
call, seeded from saved swaps), and have startReverse retry with the next index
when Boltz reports the preimage already exists — which also recovers wallets
already stuck on a burned index.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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