Fix SP scanning jank: memoize by tweak + yield + coalesce

Profiling (CDP) showed the toggle lag/high CPU was _scanSpMempool re-scanning the
ENTIRE mempool item set on every WS push (the mempool churns constantly), redoing
an EC point-multiplication per tweak on the main thread — ~1s blocks.

- Memoize silentPaymentScan/silentPaymentCandidate by tweak (outcome is fixed),
  cleared when the SP keys change, capped to bound memory. Steady-state CPU from
  re-scanning dropped from ~1s/push to ~0 (measured 18ms over 9s idle).
- Yield to the event loop during the catch-up block loop and the mempool scan, so
  a large first/cold scan (mainnet) doesn't freeze the UI.
- Coalesce mempool scans (latest-wins, never two at once) with a race guard so a
  block confirming mid-scan can't duplicate an outpoint.

Measured after: receive toggle 8/8 switches, avg 5ms / max 9ms (was 1-2s).

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

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