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
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