Propagate VCD data parse errors

[raptor1820]

Summary

• Make next_vcd_change return an io::Result so VCD parser errors in the data section are surfaced instead of being treated like EOF.
• Propagate read errors through wavecat and wavediff, including threaded multi-reader paths.
• Add a malformed VCD fixture plus CLI tests for wavecat and wavediff error exits.

Why

Malformed VCD data could previously be swallowed during signal-change iteration. In practice, an invalid command in the data section could make the tools stop reading early and exit as if the file had been processed successfully.

This change ensures parser failures are reported to callers and reflected in CLI exit behavior.

Testing

• cargo test malformed_vcd_data --test diff_test
• cargo test --test diff_test

[raptor1820]

Fixed clippy errors.

[toddstrader]

Great, thanks for the fix and the tests.