Sync silent-payment scan across devices so the catch-up runs once

SP state (lastSpScan watermark + found spUtxos) was in the nostr snapshot
but, unlike Ark/gifts, its cache extension wasn't mergeAlways — so it only
applied via the newest full snapshot at load time, last-writer-wins. The
watermark could regress (a device behind on SP, or mid-rescan, publishing a
newer snapshot would reset another device's watermark), so every device
tended to re-grind the ~1M-tweak backlog independently.

Make it a commutative merge like Ark:
- mergeAlways: true, so it merges live and from older snapshots too.
- lastSpScan takes the MAX (monotonic high-water mark, never regresses) —
  this is what makes the block catch-up run once across all devices.
- spUtxos is an additive union by outpoint; a coin we already track is never
  mutated, and spend detection stays each device's own job (_refreshSpUtxos
  drops backend-spent coins), so stale merged-in coins self-heal on refresh.
- Anti-entropy: republish our superset when we know more, guarded so it stops
  once devices converge.

A manual rescan still works — it forces from=1 synchronously (no await before
capturing it), so a concurrent max-merge can't sabotage the re-scan.

Verified with a merge unit test driving the real extension (max/union/converge).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

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