Whisper audio transcription

GPU-backed audio transcription provider implementation. Whisper is currently retired from the anonymous stable x402 edge while its backing scanner service is stopped. Marketplace buyers should not treat this as an externally claimable x402 route until provider recovery and a fresh paid canary are complete. Even after provider recovery, only after GET /market.json reports paid execution enabled and the owner approves spend may a paid buyer use the retained execute/invoke/x402 path. Unsigned direct POSTs to this implementation route return marketplace_dispatch_required.