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Add fetch_with_receipt and a persistible purchase context so buyers retain a facilitator receipt beside the original HTTP response and reuse the same authorization after restart or transport loss. Cover Arc exact USDC/EURC and native Hedera USDC; a confirmed payment can coexist with merchant HTTP 500.
Expose receipt models, Ed25519 verification through the optional [receipts] extra, private lookup, and FastAPI/header propagation. Preserve existing fetch APIs and unknown receipt fields. Document trusted-key handling and recovery limits; bump to 0.88.0.
Validation: 1,236 tests pass, including shared Rust/TypeScript signature vectors, tampering, persistence failure, lost response/restart and FastAPI HTTP 500 propagation; wheel/sdist build and twine check pass. Live EURC acceptance remains pending.