Brandon Lucas 9511957076 feat: implement resumable effect handlers ...

Add support for the `resume(value)` expression in effect handler
bodies. When resume is called, the value becomes the return value
of the effect operation, allowing handlers to provide values back
to the calling code.

Implementation:
- Add Resume(Value) variant to EvalResult
- Add in_handler_depth tracking to Interpreter
- Update Expr::Resume evaluation to return Resume when in handler
- Handle Resume results in handle_effect to use as return value
- Add 2 tests for resumable handlers

Example usage:
```lux
handler prettyLogger: Logger {
    fn log(level, msg) = {
        Console.print("[" + level + "] " + msg)
        resume(())  // Return Unit to the call site
    }
}
```

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

2026-02-13 09:54:51 -05:00

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