REPL main panics on spawn/join unwrap

[APConduct]

Summary

REPL binary currently panics on startup and shutdown because main uses unwrap() for both thread spawn and join.

Reproduction

1. Trigger an error path in Repl::run().
2. Start the veld REPL binary.
3. Observe panic due to unwrap().

Expected

Errors should be surfaced cleanly and process should exit with a controlled error.

Actual

main panics on thread spawn/join unwrap paths.

Proposed fix

Return errors from spawn/join and propagate them from main with VeldError::RuntimeError.