feat: IETF Payment client support

[TheCryptoDonkey]

Summary

• Adds client-side support for WWW-Authenticate: Payment challenges (draft-ryan-httpauth-payment-01)
• Parses IETF Payment challenges from multi-scheme WWW-Authenticate headers (handles L402 + Payment coexistence)
• Decodes Lightning charge requests to extract BOLT11 invoice, payment hash, and amount
• Pays via existing wallet infrastructure (NWC, Cashu, human) — no new wallet code needed
• Builds base64url credential for Authorization: Payment header that toll-booth can verify
• Slots into fetch handler after xcashu/x402, before L402 (priority: cheapest first)
• 22 new tests across parse, credential, and integration

Test plan

• Parse tests: standard challenge, multi-scheme headers, missing fields, corrupted base64url, non-lightning methods
• Credential tests: round-trip encode/decode, optional fields, toll-booth compatibility
• All 483 existing tests pass (no regressions)
• Manual test against satgate.trotters.dev dual-scheme endpoint

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