Phase 1.5: add GraphQL example contract + mock fixture

[ToryMic]

Goal

Add a second runnable example contract so adopters can see SchemaLock against a
Stellar-flavoured GraphQL backend, not just the escrow REST mock.

Scope

• Add examples/graphql_example.yaml: a contract for a small mock GraphQL
  backend with at least one query endpoint and one mutation endpoint.
• Add examples/graphql_mock.py: a FastAPI/Starlette app serving /graphql
  with a fixed error envelope (e.g. errors[{message, path, extensions.code}]).
• Wire the new example into the Python and Rust parity fixtures so
  scripts/parity_check.py covers it too.
• Update README.md "What it checks" and the quick-start to reference the new
  example.

Why

Phase 1 currently ships a single escrow REST example. Stellar Wave backends
increasingly expose GraphQL endpoints (per stellarmarket-labs and Talenttrust
patterns), and the PRD calls GraphQL error-shape validation a Phase 2 goal — a
GraphQL example fixture de-risks that work and shows the tool working on the
shape Wave teams actually use.

Acceptance criteria

• examples/graphql_example.yaml + examples/graphql_mock.py run standalone
• schemalock test --config examples/graphql_example.yaml --base-url <url> passes locally
• Parity gate (scripts/parity_check.py) covers the new fixture; Python and Rust agree byte-for-byte
• README quick-start mentions the GraphQL example