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>

apps/tests/joinir_if_merge_multiple.hako

@@ -0,0 +1,73 @@
+// Phase 33-7: IfMerge lowering test (multiple variables pattern)
+//
+// Pattern: if cond { x=a; y=b; z=c } else { x=d; y=e; z=f } return x+y+z
+// Expected:
+//   - cond=true → (x=10, y=20, z=30) → return 60
+//   - cond=false → (x=40, y=50, z=60) → return 150
+
+static box IfMergeTest {
+    multiple_true() {
+        local x
+        local y
+        local z
+
+        if true {
+            x = 10
+            y = 20
+            z = 30
+        } else {
+            x = 40
+            y = 50
+            z = 60
+        }
+
+        return x + y + z
+    }
+
+    multiple_false() {
+        local x
+        local y
+        local z
+
+        if false {
+            x = 10
+            y = 20
+            z = 30
+        } else {
+            x = 40
+            y = 50
+            z = 60
+        }
+
+        return x + y + z
+    }
+
+    main() {
+        local result_true
+        local result_false
+
+        result_true = me.multiple_true()
+        result_false = me.multiple_false()
+
+        print("multiple_true: ")
+        print(result_true)
+        print("\n")
+
+        print("multiple_false: ")
+        print(result_false)
+        print("\n")
+
+        // Verify results
+        if result_true == 60 {
+            print("PASS: multiple_true\n")
+        } else {
+            print("FAIL: multiple_true\n")
+        }
+
+        if result_false == 150 {
+            print("PASS: multiple_false\n")
+        } else {
+            print("FAIL: multiple_false\n")
+        }
+    }
+}

apps/tests/joinir_if_merge_simple.hako

@@ -0,0 +1,67 @@
+// Phase 33-7: IfMerge lowering test (simple pattern)
+//
+// Pattern: if cond { x=a; y=b } else { x=c; y=d } return x+y
+// Expected:
+//   - cond=true → (x=1, y=2) → return 3
+//   - cond=false → (x=3, y=4) → return 7
+
+static box IfMergeTest {
+    simple_true() {
+        local x
+        local y
+
+        if true {
+            x = 1
+            y = 2
+        } else {
+            x = 3
+            y = 4
+        }
+
+        return x + y
+    }
+
+    simple_false() {
+        local x
+        local y
+
+        if false {
+            x = 1
+            y = 2
+        } else {
+            x = 3
+            y = 4
+        }
+
+        return x + y
+    }
+
+    main() {
+        local result_true
+        local result_false
+
+        result_true = me.simple_true()
+        result_false = me.simple_false()
+
+        print("simple_true: ")
+        print(result_true)
+        print("\n")
+
+        print("simple_false: ")
+        print(result_false)
+        print("\n")
+
+        // Verify results
+        if result_true == 3 {
+            print("PASS: simple_true\n")
+        } else {
+            print("FAIL: simple_true\n")
+        }
+
+        if result_false == 7 {
+            print("PASS: simple_false\n")
+        } else {
+            print("FAIL: simple_false\n")
+        }
+    }
+}