test: add integration tests and expand examples

[darthfork]

Summary

• add a dedicated tests/ tree with integration coverage for zig_binary, zig_library, and zig_test
• add test-only Starlark smoke helpers for executable/build verification
• add multi-package and Bzlmod consumer examples
• pass the selected exec-platform Zig target explicitly to build/test actions, fixing fresh macOS Zig links

Closes #13
Closes #14

Test Plan

• bazel test //...
• bazel build //examples/...