refactor(joinir): Replace magic numbers with named constants

Phase 179-A Step 3: Improve code maintainability by replacing hardcoded
magic values with descriptive named constants.

Changes:
- instruction_rewriter.rs: K_EXIT_FUNC_NAME constant for "join_func_2"
- pattern3_with_if_phi.rs: PATTERN3_K_EXIT_SUM_FINAL_ID for ValueId(18)

Benefits:
- Self-documenting code (names explain the meaning)
- Easier to maintain (change in one place)
- Prevents typos and inconsistencies

src/mir/builder/control_flow/joinir/patterns/pattern3_with_if_phi.rs

@@ -5,6 +5,10 @@ use crate::mir::builder::MirBuilder;
 use crate::mir::ValueId;
 use super::super::trace;
 
+/// Phase 179-A: Expected ValueId for k_exit parameter (sum_final) in Pattern 3
+/// This corresponds to the exit PHI input in the JoinIR lowering for loop_with_if_phi_minimal
+const PATTERN3_K_EXIT_SUM_FINAL_ID: ValueId = ValueId(18);
+
 /// Phase 194: Detection function for Pattern 3
 ///
 /// Phase 192: Updated to structure-based detection
@@ -116,8 +120,8 @@ impl MirBuilder {
                 // The loop variable is handled separately via boundary.loop_var_name
                 LoopExitBinding {
                     carrier_name: "sum".to_string(),
-                    join_exit_value: ValueId(18),  // k_exit's parameter (sum_final)
-                    host_slot: sum_var_id,          // variable_map["sum"]
+                    join_exit_value: PATTERN3_K_EXIT_SUM_FINAL_ID,  // k_exit's parameter (sum_final)
+                    host_slot: sum_var_id,                            // variable_map["sum"]
                 }
             ])
             .with_loop_var_name(Some(loop_var_name.clone()))  // Phase 33-16: Enable header PHI generation for SSA correctness