feat(joinir): Task 200-3 - JoinIRVerifier for LoopHeader PHI and ExitLine contracts

Debug-only verification module to catch JoinIR contract violations early:
- verify_loop_header_phis: Checks loop_var_name → PHI exists in header block
- verify_exit_line: Checks exit_bindings → values exist, exit block in range
- verify_joinir_contracts: Main entry point, runs all checks

Implementation:
- Added verification functions to merge/mod.rs (private module has type access)
- Called from merge_joinir_mir_blocks after exit block setup
- Only active in debug builds (#[cfg(debug_assertions)])

Benefits:
- Catches "动くけど header PHI 無い" bugs immediately
- Validates exit_bindings before variable_map reconnection
- Prevents silent contract violations during development

src/mir/join_ir/mod.rs

@@ -46,6 +46,8 @@ pub use lowering::{
 // Re-export verification functions
 pub use verify::verify_progress_for_skip_ws;
 
+// Phase 200-3: Contract verification functions are in merge/mod.rs (private module access)
+
 /// 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);