feat(joinir): Phase 33-7 IfMerge lowering for multiple-variable PHI

Implements IfMerge instruction lowering to support multiple variables
merging from if/else branches (Phase 33-7: return pattern only).

## Changes

- Add src/mir/join_ir/lowering/if_merge.rs (232 lines)
  - IfMergeLowerer with pattern matching for common variables
  - extract_written_vars() / find_written_value() helpers
  - Phase 33-7 constraint: return pattern only (k_next=None)

- Update src/mir/join_ir/lowering/mod.rs
  - Unified entry point: try_lower_if_to_joinir()
  - Priority: IfMerge → Select → if_phi fallback
  - Add IfMergeTest.* to whitelist

- Add unit tests in src/tests/mir_joinir_if_select.rs
  - test_if_merge_simple_pattern (2 variables)
  - test_if_merge_multiple_pattern (3 variables)
  - All 7/7 tests PASS ✅

- Add reference test cases
  - apps/tests/joinir_if_merge_simple.hako (2-var pattern)
  - apps/tests/joinir_if_merge_multiple.hako (3-var pattern)

## Test Results

✅ Simple pattern (2 vars): merges=2, k_next=None
✅ Multiple pattern (3 vars): merges=3, k_next=None
✅ test result: ok. 7 passed; 0 failed

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

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

src/mir/join_ir/lowering/mod.rs

@@ -19,6 +19,7 @@ pub mod exit_args_resolver;
 pub mod funcscanner_append_defs;
 pub mod funcscanner_trim;
 pub mod generic_case_a;
+pub mod if_merge; // Phase 33-7
 pub mod if_select; // Phase 33
 pub mod loop_form_intake;
 pub mod loop_scope_shape;
@@ -46,29 +47,35 @@ pub use stageb_funcscanner::lower_stageb_funcscanner_to_joinir;
 use crate::mir::join_ir::JoinInst;
 use crate::mir::{BasicBlockId, MirFunction};
 
-/// Phase 33-3: Try to lower if/else to JoinIR Select instruction
+/// Phase 33-7: Try to lower if/else to JoinIR Select/IfMerge instruction
 ///
 /// Scope:
 /// - Only applies to whitelisted functions:
 ///   - IfSelectTest.* (Phase 33-2/33-3)
+///   - IfMergeTest.* (Phase 33-7)
 ///   - JsonShapeToMap._read_value_from_pair/1 (Phase 33-4 Stage-1)
 ///   - Stage1JsonScannerBox.value_start_after_key_pos/2 (Phase 33-4 Stage-B)
 /// - Requires NYASH_JOINIR_IF_SELECT=1 environment variable
 /// - Falls back to traditional if_phi on pattern mismatch
 ///
-/// Returns Some(JoinInst::Select) if pattern matched, None otherwise.
+/// Pattern selection:
+/// - 1 variable → Select
+/// - 2+ variables → IfMerge
+///
+/// Returns Some(JoinInst::Select) or Some(JoinInst::IfMerge) if pattern matched, None otherwise.
 pub fn try_lower_if_to_joinir(
     func: &MirFunction,
     block_id: BasicBlockId,
     debug: bool,
 ) -> Option<JoinInst> {
-    // dev トグルチェック
+    // 1. dev トグルチェック
     if !crate::config::env::joinir_if_select_enabled() {
         return None;
     }
 
-    // Phase 33-4/33-5: 関数名ガード拡張（テスト + Stage-1/Stage-B 候補）
+    // 2. Phase 33-7: 関数名ガード拡張（テスト + Stage-1/Stage-B 候補）
     let is_allowed = func.signature.name.starts_with("IfSelectTest.")
+        || func.signature.name.starts_with("IfMergeTest.") // Phase 33-7
         || func.signature.name.starts_with("Stage1JsonScannerTestBox.") // Phase 33-5 test
         || func.signature.name == "JsonShapeToMap._read_value_from_pair/1"
         || func.signature.name == "Stage1JsonScannerBox.value_start_after_key_pos/2";
@@ -83,10 +90,26 @@ pub fn try_lower_if_to_joinir(
         return None;
     }
 
-    // if_select lowering を試行
-    let lowerer = if_select::IfSelectLowerer::new(debug);
+    // 3. Phase 33-7: IfMerge を優先的に試行（複数変数パターン）
+    //    IfMerge が成功すればそれを返す、失敗したら Select を試行
+    let if_merge_lowerer = if_merge::IfMergeLowerer::new(debug);
 
-    if !lowerer.can_lower_to_select(func, block_id) {
+    if if_merge_lowerer.can_lower_to_if_merge(func, block_id) {
+        if let Some(result) = if_merge_lowerer.lower_if_to_if_merge(func, block_id) {
+            if debug {
+                eprintln!(
+                    "[try_lower_if_to_joinir] IfMerge lowering used for {}",
+                    func.signature.name
+                );
+            }
+            return Some(result);
+        }
+    }
+
+    // 4. IfMerge が失敗したら Select を試行（単一変数パターン）
+    let if_select_lowerer = if_select::IfSelectLowerer::new(debug);
+
+    if !if_select_lowerer.can_lower_to_select(func, block_id) {
         if debug {
             eprintln!(
                 "[try_lower_if_to_joinir] pattern not matched for {}",
@@ -96,11 +119,11 @@ pub fn try_lower_if_to_joinir(
         return None;
     }
 
-    let result = lowerer.lower_if_to_select(func, block_id);
+    let result = if_select_lowerer.lower_if_to_select(func, block_id);
 
     if result.is_some() && debug {
         eprintln!(
-            "[try_lower_if_to_joinir] if_select lowering used for {}",
+            "[try_lower_if_to_joinir] Select lowering used for {}",
             func.signature.name
         );
     }