refine(skills): lang-rust-docs-dev - expand pillar coverage (5/8)

## Summary

`lang-rust-docs-dev` has 5/8 pillar coverage and needs expansion.

## Current Coverage

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

## Refinement Checklist

- [ ] Review if this should remain standalone or merge into `lang-rust-dev`
- [ ] Add Module System section (documenting modules, re-exports, intra-doc links)
- [ ] Add Concurrency section (documenting async code, Send/Sync)
- [ ] Add Metaprogramming section (documenting macros, proc-macros)
- [ ] Run `just validate-pillars lang-rust-docs-dev` to verify coverage
- [ ] Ensure SKILL.md is under 500 lines (use progressive disclosure)

## Validation Command

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