Add ForeSeal Gate (SDK) + verify-before-act example app

[0rkz]

Re-submitting per the guidance in #276 — these are the SDK and the example app, not the paid endpoint.

Disclosure: I'm on the PayPerByte team; both repos are ours.

• Open Source & SDKs: @foreseal/gate — MIT Express middleware: x402 USDC pay-per-call (Base) in front of any endpoint, plus an EIP-712 receipt over the exact bytes served so a buying agent can verify hash + signer before acting. Scope is authenticity + tamper-evidence, not data correctness.
• Example Apps: foreseal-anchain-aml-receipt — MIT runnable demo of the verify-before-act loop on a paid mock AML screening call: tampered, forged, and missing receipts all fail closed. Runs locally with a stand-in facilitator, no real USDC.