fix(launcher): forward SIGINT/SIGTERM signals to child process (#1145)

The Node.js launcher wrapper (bin/ocr.js) was not forwarding signals
to the Go binary child process, causing orphaned processes when the
parent received SIGINT or SIGTERM.

Changes:
- Replace spawnSync with spawn for asynchronous child process handling
- Add signal handlers for SIGINT and SIGTERM that forward signals to child
- Add error event handler for spawn failures (ENOENT, EACCES)
- Properly propagate child exit status including signal-based termination
- Add integration tests to verify signal forwarding behavior

Fixes issue where Ctrl+C or kill commands left the Go process running
after the Node.js wrapper exited.

Resolves: #1142