Interface RpcCallOpts

Optional AbortSignal to cancel the request. When the signal fires abort:

• If the signal is already aborted before the call starts, the method throws BtxNetworkError immediately without sending a request.
• If the signal fires mid-fetch, the underlying fetch is aborted and the call throws BtxNetworkError (distinguishable from BtxTimeoutError — see cause for the underlying CallerAbortError).
• If the signal fires during a retry backoff sleep, the retry loop exits immediately without sending another request.

Honored across the full retry pipeline.

Caveat for BtxChallengeClient.redeem and BtxChallengeClient.redeemBatch: if the abort fires AFTER btxd has consumed the challenge (the RPC completed server-side before the local fetch was aborted), the redemption stands — the local promise rejects but btxd has already marked the proof spent. Callers handling cancellation should verify via a separate BtxChallengeClient.verify call if the post-abort state matters.