Add `zig_test`

[darthfork]

Tests are table stakes for an official ruleset and for users adopting it in real code.

Tasks:

• Design a minimal zig_test rule using Zig’s test support.
• Ensure bazel test works naturally.
• Add examples and tests for passing and failing test targets.

Acceptance criteria:

• zig_test exists and is documented.
• bazel test succeeds on repository examples.
• Rule behavior is covered by automated tests.

Parent issue: #3