test: add example parity and cross-SDK challenge vectors

[brendanjryan]

Summary

• add separate-process client/server examples for basic, charge-basic, charge-hash, and charge-fee-payer to mirror the mpp-rs sample layout while keeping the existing one-command demos
• make the basic server generate and fund a fresh merchant address on startup, closer to the Rust sample behavior
• import cross-SDK challenge ID golden vectors from the Rust/Python SDK suites and add targeted regression coverage for untested helpers
• fix opaque={} handling so empty opaque metadata is preserved in HMAC and wire-format roundtrips like the other SDKs

Testing

• GOTOOLCHAIN=auto go test ./...

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