feat(conformance): add open_mandate_hash derivation vectors v0

code/sdk/schemas/ap2/conformance/open_mandate_hash/README.md

@@ -0,0 +1,78 @@
+# AP2 `open_mandate_hash` Conformance Vectors — v0
+
+Conformance vectors for the `open_mandate_hash` derivation rule for
+`open_checkout_mandate.json`. Any AP2 implementation that derives
+`open_mandate_hash` MUST reproduce these vectors byte-for-byte.
+
+## Derivation rule
+
+```
+open_mandate_hash = SHA-256(JCS_RFC8785(unsigned_open_checkout_mandate_body))
+```
+
+**Lowercase hex output.**
+
+The hash input is the mandate **claims object**, not the JWS compact form.
+Re-encoding the JWS envelope MUST NOT change `open_mandate_hash`.
+
+## Vectors
+
+`vectors-v0.json` — 7 vectors anchored to
+`code/sdk/schemas/ap2/open_checkout_mandate.json` at schema commit
+`e3d9cafa7311d90612c7f908ae9b8821ddc8735a`.
+
+Each vector is structured as:
+
+```json
+{
+  "vector_id": "ap2-omh-v0-<name>",
+  "mandate_body": { ... },
+  "expected_jcs_bytes_b64": "<base64url of RFC 8785 canonical bytes>",
+  "expected_open_mandate_hash": "sha256:<lowercase-hex>",
+  "expectation": "reference | same_hash_as:<id> | different_hash_from:<id>"
+}
+```
+
+| Vector | Pair invariant | Tests |
+|---|---|---|
+| `baseline-001` | reference | Canonical baseline |
+| `object-key-order-002` | `same_hash_as:baseline-001` | JCS sorts object members |
+| `array-order-003` | `different_hash_from:baseline-001` | Arrays are order-significant |
+| `optional-fields-004` | `different_hash_from:baseline-001` | Presence ≠ absence |
+| `currency-minor-unit-005` | canonical form | Integer minor units only |
+| `unicode-nfc-006a` | `different_hash_from:unicode-nfd-006b` | No Unicode normalisation |
+| `unicode-nfd-006b` | `different_hash_from:unicode-nfc-006a` | No Unicode normalisation |
+
+The array-order and Unicode pairs catch the divergences most commonly seen
+in practice: implementations that sort arrays or NFC-normalise strings will
+fail the corresponding pair invariant immediately.
+
+## Cross-implementation validation
+
+| Implementation | Language | Library | Result |
+|---|---|---|---|
+| `rfc8785@0.1.4` | Python | Trail of Bits | 7/7 + 4/4 pair invariants ✓ |
+| `canonicalize@3.0.0` | JavaScript | Erdtman + Rundgren (RFC 8785 author) | 7/7 + 4/4 ✓ |
+| `gowebpki/jcs v1.0.1` | Go | @amavashev (AP2 maintainer) | 7/7 + 4/4 ✓ |
+| `cyberphone/json-canonicalization` | Java | Rundgren (RFC 8785 reference impl) | 7/7 + 4/4 ✓ |
+| `serde_jcs 0.2.0` | Rust | @seritalien / Vauban | 7/7 + 4/4 ✓ |
+| `rfc8785` (fixed) | Python | Crest Systems | 7/7 + 4/4 ✓ |
+
+Full validation history: [AP2 issue #265](https://github.com/google-agentic-commerce/AP2/issues/265)
+
+## Known implementation hazards
+
+- **`json.dumps()` non-ASCII escaping** — Python's `json.dumps()` escapes
+  non-ASCII characters as `\uXXXX`. RFC 8785 requires literal UTF-8 bytes
+  for printable codepoints above U+007F. Use the `rfc8785` library instead.
+  This will cause failures on vectors 006a and 006b.
+
+- **Array sorting** — Do not sort arrays before hashing. JCS preserves
+  array element order. Vector 003 catches this.
+
+- **Unicode normalisation** — Do not NFC-normalise strings before hashing.
+  RFC 8785 makes no Unicode normalisation. Vectors 006a/006b catch this.
+
+- **Float/decimal prices** — `item.price` and `amount.amount` are integers
+  in the currency minor unit. `10.50` MUST be encoded as `1050`. Vector
+  005 catches this.