refine(skills): lang-rust-errors-dev - expand pillar coverage (1/8)

## Summary

`lang-rust-errors-dev` has 1/8 pillar coverage and needs significant expansion or consideration for merging.

## Current Coverage

- ✗ Module System
- ✓ Error Handling
- ✗ Concurrency
- ✗ Metaprogramming
- ✗ Zero/Default Values
- ✗ Serialization
- ✗ Build/Dependencies
- ✗ Testing

## Refinement Checklist

- [ ] Review if this should remain a standalone skill or merge into `lang-rust-dev`
- [ ] If keeping standalone, add Module System section
- [ ] If keeping standalone, add Concurrency section
- [ ] If keeping standalone, add Metaprogramming section
- [ ] If keeping standalone, add Zero/Default Values section
- [ ] If keeping standalone, add Serialization section
- [ ] If keeping standalone, add Build/Dependencies section
- [ ] If keeping standalone, add Testing section
- [ ] Run `just validate-pillars lang-rust-errors-dev` to verify coverage
- [ ] Ensure SKILL.md is under 500 lines (use progressive disclosure)

## Validation Command

```bash
just validate-pillars lang-rust-errors-dev
```