feedback(lang-roc-dev): expand Zero/Default Values section

## Feedback: lang-roc-dev

### Context
- **Task**: Creating `convert-roc-erlang` skill
- **Used for**: Understanding Roc's approach to optional values and defaults

### What Worked Well
- Tag unions for optional values are explained well
- Record update syntax is clear
- Type system fundamentals are solid

### Suggested Improvements
- [ ] Add dedicated ## Zero and Default Values section
- [ ] Document canonical empty/zero values for each type (List, Dict, Set, etc.)
- [ ] Explain default value patterns (no built-in defaults)
- [ ] Show how to handle missing/optional values idiomatically
- [ ] Cover initialization patterns for records with many fields
- [ ] Document None pattern vs explicit tag unions
- [ ] Add examples of safe access patterns

Currently, optional values are mentioned in context of tag unions, but a dedicated section would help with systematic understanding of Roc's approach to defaults.

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Feedback from: `/create-lang-conversion-skill roc erlang`