Implement the web UI server with Zig I/O

Closes #9

Summary

• implement an HTTP server on the current std.Io.net APIs and serve the embedded UI
• stream JSON-encoded test events over SSE, including flaky outcomes, while pruning disconnected clients
• make startup and shutdown own their accept thread and sockets, including partial-request cancellation
• enable --ui in discovery/watch mode and make Ctrl+C unwind through normal cleanup
• add loopback integration tests using OS-assigned ports

Validation

• zig fmt --check .
• zig build test
• zig build examples
• zig build -Dtarget=x86_64-windows
• live CLI smoke test with HTTP, SSE, and Ctrl+C cleanup