Implement zero-progress backedge detection for JoinIR loops:
- Create verify.rs module with ProgressError enum
- verify_progress_for_skip_ws() checks BinOp::Add before recursive calls
- Hook into joinir_runner_standalone_skip_ws test (NYASH_JOINIR_EXPERIMENT=1)
- Currently warning-only, will upgrade to error in Phase 30
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Co-Authored-By: Claude <noreply@anthropic.com>