Spend SP coins like any coin — confirmed-only, and able to fund SP payments

Two things:
- Only confirmed SP coins enter the spend pool. Unconfirmed ones are excluded
  (the sender's tx could be replaced, invalidating our spend) — matching the
  spendable headline, which already counted confirmed-only.
- SP→SP sending: our own silent-payment coins can now fund a payment TO an sp1
  address (previously excluded). _applySilentPayments includes SP inputs with
  their key d = spend_priv + t_k, and silentPaymentScripts negates a taproot
  input's key when it gives odd Y, per BIP-352 (so the sender's A = a·G matches
  what the indexer reconstructs). signing was already SP-aware.

Validated: BIP-352 sending vectors pass for all clean key-path-taproot-input
cases (the remaining vector failures are NUMS script-path / labels / uncompressed
keys — input types the wallet never spends). End-to-end on regtest: an SP coin
funds a payment to another sp address, testmempoolaccept passes, and the recipient
scans and finds it.

Note: silentPaymentScripts derives input_hash from the eligible inputs it's given,
which is correct because the wallet only ever spends all-eligible coins (P2WPKH +
key-path SP); it isn't a general-purpose sender for txs with ineligible inputs.

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

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