perf(python): reuse a Tokio runtime within SandboxFactory

[sd2k]

SandboxFactory.create_sandbox() reuses a precompiled artifact but constructs a new multithreaded Tokio runtime for each child sandbox. Reuse a runtime within the factory to avoid repeated worker-thread creation and teardown while preserving independently configured child sandboxes.

The public implementation shows:

• create_sandbox() calls Sandbox::from_inner().
• Sandbox::from_inner() builds a new multithreaded Tokio runtime on every call.
• create_session() also constructs a runtime per child.

Proposed approach:

• Share an Arc<tokio::runtime::Runtime> among a factory's children, covering both new and loaded factories.
• Keep child-specific callbacks, output handlers, secrets and resource limits separate.
• Preserve fresh guest Store/instance and default VFS creation for each stateless Sandbox.execute() call; this does not require reusing persistent sessions between tenants.
• Define lazy initialization, prefork/process ownership and shutdown behavior. Children must remain usable after their factory is dropped.

Validate concurrent execution, callback routing, destruction order, Python GIL interactions, error/timeout recovery and thread-count stability. Benchmark total process CPU and latency for repeated child creation/execution/teardown to quantify the benefit.

Related: #411 optimizes Wasmtime allocation and preparation of fresh instances. Sharing the Python bindings' Tokio runtime is a separate optimization.