feat(joinir): add progress carrier verification for skip_ws (Phase 29 L-5.2)

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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/join_ir/mod.rs

@@ -27,11 +27,17 @@ use crate::mir::ValueId;
 // Phase 27.9: Lowering submodule
 pub mod lowering;
 
+// Phase 29 L-5.2: Progress verification
+pub mod verify;
+
 // Re-export lowering functions for backward compatibility
 pub use lowering::{
     lower_funcscanner_trim_to_joinir, lower_min_loop_to_joinir, lower_skip_ws_to_joinir,
 };
 
+// Re-export verification functions
+pub use verify::verify_progress_for_skip_ws;
+
 /// JoinIR 関数ID（MIR 関数とは別 ID でもよい）
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct JoinFuncId(pub u32);