Architecture: define contract representation layers

[sfloess]

Summary

• define semantic, schema, conformance, and binding layers for Loom contracts
• establish JSON Schema as the initial machine-readable structural representation
• explicitly position OpenAPI as an HTTP binding, not the normative contract
• add ADR-0007 and an initial Contract schema

Why

This makes the contract representation strategy explicit before implementing loom-python. The semantic model remains transport- and language-neutral while schemas and bindings provide concrete realizations.

Closes #2 is intentionally deferred until the full normative semantic model and conformance suite are frozen; this PR establishes the representation decision needed to proceed safely.