Integrate BRC-118: multipart BEEF transport for large payment payloads

[b0ase]

Context

BRC-118 (merged as bitcoin-sv/BRCs#122) solves BEEF transaction bloat when unconfirmed ancestors accumulate during rapid sequential payments.

Problem: BEEF payloads grow as transactions chain before confirmation. Real-world measurements show single-transaction BEEFs reaching 155.7KB in 5 minutes — exceeding Cloudflare's 128KB header limit.

Solution: Negotiated transport via x-bsv-payment-transports header. When payloads exceed header limits, client sends payment via multipart/form-data body instead. Fully backward compatible with BRC-103/105.

Why this matters for $402

• Proof-of-Indexing payments chain rapidly — exactly the scenario where BEEF bloat hits
• ClawMiner devices making sequential $402 payments accumulate unconfirmed ancestors fast
• Content micropayments (pay-per-page, pay-per-stream) create long unconfirmed chains
• Without BRC-118, high-frequency $402 payments will silently fail at CDN header limits

Implementation plan

1. Add x-bsv-payment-transports header negotiation to path402 client SDK
2. Implement multipart fallback in @path402/client when BEEF exceeds threshold (~100KB)
3. Add server-side multipart parsing to path402 payment verification middleware
4. Test with ClawMiner rapid-payment scenarios

References

• BRC-118 spec
• Extends BRC-105 (payment handling) and BRC-103 (authentication)
• 3 production implementations exist (TypeScript, Go, Python)