docs: Phase 190-impl-D complete - NumberAccumulation PHI wiring fixed

- Fixed ValueId collision between body-local and carrier params
- Added ExitLine contract verifier (debug assertions)
- Updated test files to use Main box
- E2E verified: atoi→12, parse_number→123

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/merge/exit_line/reconnector.rs

@@ -145,8 +145,68 @@ impl ExitLineReconnector {
             );
         }
 
+        // Phase 190-impl-D-3: Contract verification (debug build only)
+        // Ensures all exit_bindings have corresponding entries in carrier_phis and variable_map
+        #[cfg(debug_assertions)]
+        Self::verify_exit_line_contract(boundary, carrier_phis, &builder.variable_map);
+
         Ok(())
     }
+
+    /// Phase 190-impl-D-3: Verify exit line contract (debug build only)
+    ///
+    /// # Contract Requirements
+    ///
+    /// 1. Every exit_binding must have a corresponding entry in carrier_phis
+    /// 2. Every exit_binding's carrier must exist in variable_map after reconnect
+    /// 3. The variable_map entry must point to the PHI dst (not the original host value)
+    ///
+    /// # Panics
+    ///
+    /// Panics if any contract violation is detected. This helps catch bugs where:
+    /// - PHI is missing for a carrier (Phase 190-impl-D root cause)
+    /// - variable_map update was skipped
+    /// - ValueId collision occurred
+    #[cfg(debug_assertions)]
+    fn verify_exit_line_contract(
+        boundary: &JoinInlineBoundary,
+        carrier_phis: &BTreeMap<String, ValueId>,
+        variable_map: &std::collections::HashMap<String, ValueId>,
+    ) {
+        for binding in &boundary.exit_bindings {
+            // Contract 1: carrier_phis must contain this carrier
+            let phi_dst = carrier_phis.get(&binding.carrier_name);
+            if phi_dst.is_none() {
+                // Skip loop variable (it's handled separately in loop_header_phi)
+                // Only check carriers that have exit_bindings
+                eprintln!(
+                    "[JoinIR/ExitLine/Contract] WARNING: Carrier '{}' has exit_binding but no PHI in carrier_phis",
+                    binding.carrier_name
+                );
+                // Don't panic for now - loop variable might not be in carrier_phis
+                // Future: Distinguish loop_var from carriers in exit_bindings
+            }
+
+            // Contract 2: variable_map must contain this carrier after reconnect
+            let var_value = variable_map.get(&binding.carrier_name);
+            if var_value.is_none() {
+                panic!(
+                    "[JoinIR/ExitLine/Contract] VIOLATION: Carrier '{}' missing from variable_map after reconnect",
+                    binding.carrier_name
+                );
+            }
+
+            // Contract 3: variable_map entry should point to PHI dst (if PHI exists)
+            if let (Some(&phi), Some(&var)) = (phi_dst, var_value) {
+                if phi != var {
+                    panic!(
+                        "[JoinIR/ExitLine/Contract] VIOLATION: Carrier '{}' variable_map={:?} but PHI dst={:?} (mismatch!)",
+                        binding.carrier_name, var, phi
+                    );
+                }
+            }
+        }
+    }
 }
 
 #[cfg(test)]