WDC: nonce pre-exchange for instant signing

[kwsantiago]

Problem

FROST signing requires a nonce commitment round before the signing round. For time-sensitive transactions, this adds latency. Pre-exchanging nonces during idle time enables single-round signing when needed.

Solution

Implement Bifrost-style nonce pool management in keep-frost-net.

Reference

• bifrost (src/class/pool.ts): maintains separate incoming/outgoing nonce pools, auto-replenishment via events when pools fall below threshold, marks nonces as spent immediately

Implementation

• Outgoing pool: generate nonce commitments for each peer, send during idle
• Incoming pool: store received commitments, track consumption
• Auto-replenish when pool drops below configurable threshold
• Integrate with existing signing flow — use pooled nonce if available, fall back to interactive round