Unify discovery and programmatic test execution

Summary

• add a shared TestPlan, execution policy, and typed lifecycle event stream
• route registered and discovered tests through the same result and reporter model
• support reporter selection, JUnit output, filtering, bail, and timeout semantics consistently
• document architecture, extension points, and migration guidance

Verification

• zig build test -j1 --summary none
• zig build
• zig build examples
• zig fmt --check .
• zig build-lib -femit-docs src/lib.zig

Closes #5