Add Permit2 scheme partition

[iamonuwa]

x402 supports Permit2-based payments alongside EIP-3009, via permitWitnessTransferFrom on the canonical Permit2 contract and the x402 Permit2 proxy. Permit2 is a different signing scheme, not a variant of the existing partition: its EIP-712 domain uses the canonical Permit2 contract as verifying contract rather than the token, it signs a PermitTransferFrom struct instead of TransferWithAuthorization, it uses unordered nonce bitmaps instead of 32-byte random nonces, and it carries a single deadline instead of a validAfter/validBefore window. It therefore enters the corpus as a new scheme partition with its own vector namespace, reason-code mapping, and generator path. kanon-gen and kanon-core must not share the new EIP-712 plumbing, per the independent-implementations rule.

Candidate failure modes, each needing cited provenance before a vector exists:

• wrong Permit2 domain (chainId or verifying contract), resolving to SIGNER_MISMATCH.
• nonce bitmap replay, the Permit2 analogue of NONCE_REPLAY.
• expired deadline, mapping to EXPIRED.
• signed amount below required amount, AMOUNT_INSUFFICIENT.

Notes on codes that do not carry over cleanly:

• SIG_MALLEABLE does not apply as-is. Permit2's SignatureVerification does not enforce the EIP-2 low-s rule and accepts high-s signatures that recover correctly. If covered at all, it is as a documented divergence from the EIP-3009 lane, not a shared rejection rule.
• Base Permit2 has no start time, so there is no NOT_YET_VALID analogue at the Permit2 layer. The x402 proxy's witness struct does carry a validAfter, so the code may reappear at the proxy layer. To be settled in the spec work.

Open questions to settle in the spec work:

• whether verification targets the bare Permit2 signature or the proxy witness structure, which determines the reason-code surface.
• how the vector id and encodes conventions extend to the new partition.