test(rfq): prove confidential P2TR settlement

[tvolk131]

Summary

• add deterministic offline coverage for a collaboratively blinded and signed confidential P2TR RFQ settlement between two wallets
• add a live liquidregtest gate that issues distinct payment and outcome assets, rejects unilateral settlement, mines the complete swap, and proves both recipients can spend what they received
• validate exact settlement intent, disclosure proofs, consensus proofs, signatures, and recipient-specific output recoverability across serialized PSET handoffs
• wire the new gate into just regtest and document the focused command

Why

This completes the next RFQ-first roadmap milestone: prove the underlying two-wallet confidential settlement mechanics before freezing a production venue API or remote RFQ protocol. The implementation is deliberately test-local.

The test keeps the policy asset separate from the traded payment and outcome assets, exercises non-adjacent RFQ inputs and outputs, and includes fail-closed mutations. It also covers the ECDH-nonce case where public proofs and consensus validation can succeed while the intended recipient cannot unblind the output.

Validation

• nix develop .#default --command just ci-checks
  • 213 tests passed, 6 skipped
  • documentation tests passed
  • WASM check passed
• nix develop .#default --command just regtest-rfq-settlement
• strict Clippy for the new integration test
• git diff --check

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