Pin API contract 1.0.0 and check every call against it

CONTRACT_VERSION

@@ -0,0 +1 @@
+1.0.0

README.md

@@ -17,6 +17,8 @@ go get github.com/livexface/livexface-go
 
 Requires Go 1.22 or later.
 
+Validated against API contract 1.0.0 (`/openapi.json` `info.version`), exposed as `livexface.ContractVersion`. The test suite calls every client method against the pinned contract in `contract/` and fails if a method or path is missing from it or a required field is not sent; to move to a new contract, copy the release asset `openapi-<version>.json` into `contract/` and update `CONTRACT_VERSION` and the `ContractVersion` constant.
+
 ## Quick Start
 
 ```go

client.go

@@ -20,6 +20,11 @@ import (
 
 const defaultBaseURL = "https://api.livexface.com/api/v1"
 
+// ContractVersion is the API contract version (info.version of /openapi.json)
+// this SDK release is validated against. The contract is pinned in
+// contract/openapi-<version>.json and checked by the test suite.
+const ContractVersion = "1.0.0"
+
 // ─── Error ────────────────────────────────────────────────────────────────────
 
 // APIError is returned when the server responds with a non-successful payload.