Add a real `tests/` tree

[darthfork]

A ruleset needs more than a hello-world example. It needs regression coverage.

Tasks:

• Create tests/ with analysis and integration coverage.
• Add rule behavior tests for zig_binary, zig_library, and future zig_test.
• Add Bzlmod and module-extension coverage.
• Add negative tests for invalid attrs and missing toolchain configuration.

Acceptance criteria:

• A dedicated tests/ directory exists.
• Core rules and extension behavior are covered.
• CI runs tests on every change.

Parent issue: #4