fix(unix): validate spawn-helper exists before calling pty.fork()

[theJC]

Problem

`helperPath` is computed once at module load time using `__dirname`:

```ts
let helperPath = native.dir + '/spawn-helper';
helperPath = path.resolve(__dirname, helperPath);
```

If the application binary moves after the process starts — for example, a dev build whose repository directory is relocated while the app is running — `helperPath` becomes stale. On the next call to `pty.fork()`, `posix_spawn` receives a non-existent path for `argv[0]` and returns `ENOENT`.

Previously this had two bad outcomes:

1. Opaque error: the native layer surfaced `"posix_spawnp failed."` with no path information, making diagnosis difficult.
2. Process crash: in some Electron/V8 versions, the `Napi::Error` thrown from the failed spawn path left V8 heap objects in an inconsistent state. A GC pass ~4 seconds later hit a `CHECK` failure and aborted the process with `EXC_BREAKPOINT (SIGTRAP)` — turning a recoverable error into a full application crash.

Fix

Add an `fs.existsSync` guard in the `UnixTerminal` constructor before calling `pty.fork()`. This:

• Throws a clean JavaScript error before entering native code, keeping the process alive
• Includes the resolved `helperPath` in the error message for immediate diagnosis
• Is essentially free (one `stat` syscall) and only fails in the narrow case where the binary has moved

Related

• The fd leak in `pty_posix_spawn` that contributed to the V8 heap corruption is already fixed in `main` via fix: /dev/ptmx leak on macOS #882.
• This PR addresses the remaining gap: the JS layer never validates that the helper binary is reachable before delegating to native code.

Testing

Unit test added in `src/unixTerminal.test.ts` under the existing `darwin`-only block in `describe('spawn')`. It temporarily renames the `spawn-helper` binary, asserts the constructor throws with a message containing both `"spawn-helper not found"` and the resolved path, then restores the binary in a `finally` block — the same rename-and-restore pattern used by the ptmx fd leak test added in #882. No additional dependencies required.

Manual testing via the Electron example (`examples/electron`) confirmed:

• Happy path: normal `pty.spawn` works correctly
• Error path: with `spawn-helper` renamed, the constructor throws a clean JS error and the process stays alive

[theJC]

Tested manually per CONTRIBUTING.md using the Electron example (examples/electron):

• Happy path: normal pty.spawn worked correctly in the Electron terminal
• Error path: temporarily renamed spawn-helper binary; constructor threw a clean JS error with the full path included, and the process stayed alive (no native crash)

[theJC]

Added a unit test in src/unixTerminal.test.ts that temporarily renames the spawn-helper binary, verifies the constructor throws with a message containing both spawn-helper not found and the resolved path, then restores the file. Runs under the existing darwin-only block in describe('spawn'). All 24 tests pass.

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