Phase 1: Add matrix strategy for Rust versions

[RAprogramm]

Task

Test on multiple Rust versions (MSRV, stable) using matrix strategy.

Implementation

strategy:
  fail-fast: false
  matrix:
    rust: 
      - ${{ needs.msrv.outputs.msrv }}
      - stable

Benefits

• Catch version-specific bugs
• Ensure MSRV compatibility
• Test against latest stable

Acceptance Criteria

• Matrix strategy added to CI
• Tests run on MSRV
• Tests run on stable
• Both versions must pass

Parent: #175