NodeBridge dispose should cancel in-flight init to avoid orphaned processes

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Summary

Disposing a NodeBridge while init() is in flight can leave a Python subprocess running because dispose() returns early when child is still unset, but startProcess() later assigns this.child.

Failure mode

• Trigger bridge.init() (or bridge.call() which calls init()), then call dispose() before startProcess() assigns this.child.
• dispose() sees child as undefined, exits without killing anything.
• startProcess() completes and leaves a live Python subprocess despite disposed=true.

Evidence

• src/runtime/node.ts in init() and startProcess().

Proposed fix

• Add a disposal guard inside startProcess() after spawn: if this.disposed, kill the child immediately and bail.
• Optionally track a cancel token/promise so dispose() can await startProcess() and kill the child deterministically.

Acceptance criteria

• Disposing during init leaves no running Python subprocess.
• Subsequent calls after dispose fail with BridgeDisposedError and do not leak processes.
• Add a regression test that triggers dispose mid-init.

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Resolved by PR #150. BoundedContext guards init() against 'disposing' state, preventing init during disposal.