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/merge/instruction_rewriter.rs

@@ -5,6 +5,7 @@
 //!
 //! Phase 4 Extraction: Separated from merge_joinir_mir_blocks (lines 260-546)
 //! Phase 33-17: Further modularization - extracted TailCallClassifier and MergeResult
+//! Phase 179-A Step 3: Named constants for magic values
 
 use crate::mir::{BasicBlock, BasicBlockId, MirInstruction, MirModule, ValueId};
 use crate::mir::builder::joinir_id_remapper::JoinIrIdRemapper;
@@ -14,6 +15,10 @@ use super::tail_call_classifier::{TailCallKind, classify_tail_call};
 use super::merge_result::MergeResult;
 use std::collections::HashMap;
 
+/// Phase 179-A: Exit continuation function name (MIR convention)
+/// This is the standard name for k_exit continuations in JoinIR → MIR lowering
+const K_EXIT_FUNC_NAME: &str = "join_func_2";
+
 /// Phase 4: Merge ALL functions and rewrite instructions
 ///
 /// Returns:
@@ -88,8 +93,8 @@ pub(super) fn merge_and_rewrite(
         // Phase 33-15: Identify continuation functions (join_func_2 = k_exit, etc.)
         // Continuation functions receive values from Jump args, not as independent sources
         // We should NOT collect their Return values for exit_phi_inputs
-        // Note: MIR uses "join_func_N" naming, where N=2 is typically k_exit
-        let is_continuation_func = func_name == "join_func_2" || func_name.ends_with("k_exit");
+        // Phase 179-A: Use named constant for k_exit function name
+        let is_continuation_func = func_name == K_EXIT_FUNC_NAME || func_name.ends_with("k_exit");
 
         if debug {
             eprintln!(