Add Parity loop analysis showcase

[Maokami]

Goal

Add an end-to-end loop showcase for the Parity domain, mirroring the existing Sign (LoopShowcase) and Interval (WidenShowcase) patterns.

Scope

• Create Examples/IMP/Programs/ParityLoopShowcase.lean
• Demonstrate: loop + widening + post-fixpoint soundness with Parity domain
• Shows that generic analysis code works across domains without modification

Acceptance Criteria

• Parity loop showcase compiles and proves a concrete property
• Demonstrates generic domain reuse (same analysis pipeline, different domain)
• lake build && lake build Tests && lake test pass