feat(runtime): add shutdown() method to gracefully stop all boxes

[DorianZheng]

Summary

Implement runtime.shutdown() across all SDKs to gracefully stop all running boxes with configurable timeout.

• Add boxes_stopped counter to track stopped boxes
• Implement num_running_boxes() calculation: created - stopped - failed
• Make box.stop() idempotent (safe to call multiple times)
• Add shutdown() method to Python, Node.js, and C SDKs

API

# Python
await runtime.shutdown()          # Default 10s timeout
await runtime.shutdown(timeout=5) # Custom timeout
await runtime.shutdown(timeout=-1) # Wait indefinitely

// Node.js
await runtime.shutdown();    // Default 10s timeout
await runtime.shutdown(5);   // Custom timeout
await runtime.shutdown(-1);  // Wait indefinitely

// C
boxlite_runtime_shutdown(runtime, 0, &error);  // Default (0 = 10s)
boxlite_runtime_shutdown(runtime, 5, &error);  // Custom timeout
boxlite_runtime_shutdown(runtime, -1, &error); // Wait indefinitely

Files Changed

Category	Files
Core	box_impl.rs, runtime_metrics.rs
C SDK	boxlite.h, ffi.rs
Node.js SDK	runtime.rs
Python SDK	runtime.rs, _boxlite.py
Examples	shutdown.c, shutdown.js, shutdown_example.py
Tests	shutdown.test.ts, test_shutdown.py

Test plan

• Run Rust unit tests: cargo test -p boxlite --lib
• Run Python integration tests: pytest sdks/python/tests/test_shutdown.py -v
• Run Node.js integration tests: cd sdks/node && npm test
• Run examples manually to verify behavior