feat(joinir): Phase 172-3 ExitMeta unified return from Pattern2 lowerer

Implement loop exit contract boxification for JoinIR Pattern2:

- lower_loop_with_break_minimal now returns (JoinModule, ExitMeta)
- ExitMeta contains k_exit parameter ValueId for carrier variables
- Pattern2 caller builds exit_bindings from ExitMeta
- merge/mod.rs adds exit_bindings join_exit_values to used_values for remap
- reconnect_boundary uses remapped exit values for variable_map updates

This completes Phases 172-3 through 172-5 of the Loop Exit Contract
boxification plan, enabling proper loop variable propagation after exit.

Test: joinir_min_loop.hako passes (RC: 0)

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

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

src/mir/builder/control_flow/joinir/merge/mod.rs

@@ -88,6 +88,17 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
             }
             used_values.insert(binding.join_value);
         }
+
+        // Phase 172-3: Add exit_bindings' join_exit_values to used_values for remapping
+        for binding in &boundary.exit_bindings {
+            if debug {
+                eprintln!(
+                    "[cf_loop/joinir] Phase 172-3: Adding exit binding '{}' JoinIR {:?} to used_values",
+                    binding.carrier_name, binding.join_exit_value
+                );
+            }
+            used_values.insert(binding.join_exit_value);
+        }
     }
 
     // Phase 3: Remap ValueIds