Run live Node SDK execution in process

### Description of the new feature / enhancement

Move the existing live Node SDK execution APIs in process while preserving their current stream-based interface.

`spawnSandbox` and `spawnSandboxFromConfig` should use `mxc_ffi` rather than launching an executor binary. Callers must retain live stdin, stdout, stderr, cancellation, timeout, and terminal metadata behavior without exposing native handles publicly.

**Success criteria**

- Existing live spawn APIs execute through the shared native library.
- Node streams own and release native stream handles safely.
- Blocking native I/O does not block the Node event loop.
- Cancellation and timeout interrupt pending output reads.
- Native request and handle details remain private.

Part of #1168.

### Proposed technical implementation details

- Bind the existing `mxc_ffi` streaming functions through Koffi.
- Adapt native handles behind the existing `MxcSandboxProcess` API.
- Use Koffi's asynchronous call surface for native stream I/O.
- Keep handle acquisition, interruption, waiting, and disposal in separate internal layers.

Start by locating the existing spawnSandbox and spawnSandboxFromConfig entry points and the MxcSandboxProcess API. Read the mxc_ffi streaming functions and Koffi's asynchronous call surface, then trace handle acquisition, interruption, waiting, and disposal. Done means live execution uses the shared native library while preserving streams, cancellation, timeout, terminal metadata, and event-loop responsiveness without exposing native handles.

Written by the indexing model from the issue text.