docs(x402): align runner docs with restricted auth and dry-run behavior

[drewstone]

Summary

Adds the x402 runner docs page and aligns it with the merged x402 hardening behavior in blueprint.

What this documents

• Explicit per-job invocation policy model:
  • disabled
  • public_paid
  • restricted_paid
• Restricted authorization modes:
  • payer_is_caller
  • delegated_caller_signature
• On-chain parity checks for restricted jobs via isPermittedCaller(service_id, caller) (eth_call).
• Delegated signature headers and payload contract, including nonce:
  • X-TANGLE-CALLER
  • X-TANGLE-CALLER-SIG
  • X-TANGLE-CALLER-NONCE
  • X-TANGLE-CALLER-EXPIRY
  • payload: x402-authorize:{service_id}:{job_index}:{keccak(body)}:{nonce}:{expiry}
• New gateway endpoints:
  • GET /x402/stats
  • POST /x402/jobs/{service_id}/{job_index}/auth-dry-run
• Runtime semantics:
  • 202 Accepted means accepted/enqueued (not final completion)
  • replay denial behavior (409 signature_replay)
  • dry-run is non-mutating and does not consume delegated nonces

Notes

This PR is intentionally scoped to x402 docs only.

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