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

[aRustyDev]

Summary

lang-rust-cargo-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 Error Handling section (cargo errors, dependency resolution failures)
• Add Concurrency section (parallel builds, cargo jobs)
• Add Metaprogramming section (build scripts, proc-macro crates, code generation)
• Run just validate-pillars lang-rust-cargo-dev to verify coverage
• Ensure SKILL.md is under 500 lines (use progressive disclosure)

Validation Command

just validate-pillars lang-rust-cargo-dev