`supports_interface` implementations return true without interface validation

[therealjhay]

ProblemBoth contracts' supports_interface always return true for any version; the interface contract isn't enforced anywhere (issue 48).

1. Always-true stub.
2. No version table.
3. Meaningless API.

Solution

• Implement version semantics or document the placeholder.
• Test.

Acceptance Criteria

• Interface behavior defined and tested.
• cargo test --workspace passes.

Note for Contributors: Write a clear PR description. Show the interface test.