feat(phi): Phase 25.1 - LoopForm v2 テスト・最小再現ケース追加

- ✅ 最小再現ケース作成
  - apps/tests/minimal_ssa_skip_ws.hako: 確実に再現する10-30行ケース
  - apps/tests/minimal_ssa_bug*.hako: 段階的簡略化版
  - apps/tests/loopform_*.hako: LoopForm v2 各ケーステスト

- ✅ Rustテストスイート追加
  - src/tests/mir_loopform_conditional_reassign.rs: 4ケース（Case A/B/C/D）
  - src/tests/mir_loopform_complex.rs: 複雑なパターン
  - 全テストPASS確認済み

- ✅ SSAバグ分析ドキュメント
  - docs/development/analysis/minimal_ssa_bug_analysis.md
  - エラー詳細・原因・ワークアラウンド記録

🎯 成果: SSAバグの構造を完全特定、デバッグ準備完了

apps/tests/loopform_break_and_return.hako

@@ -0,0 +1,26 @@
+// loopform_break_and_return.hako
+// 目的: header-cond + break + early-return が混在するループをスモークする
+
+static box Main {
+  compute(limit) {
+    local i = 0
+    // Case A: header-cond (i < limit)
+    loop(i < limit) {
+      if i == 5 { return -1 }      // 早期 return（exit PHI 非対象）
+      if i >= limit - 1 { break }  // exit-break
+      i = i + 1
+    }
+    return i
+  }
+
+  main(args) {
+    // limit が小さい場合は break 経路で終了
+    local r1 = Main.compute(3)
+    print(r1)
+    // limit が大きい場合は i==5 で早期 return
+    local r2 = Main.compute(10)
+    print(r2)
+    return 0
+  }
+}
+

apps/tests/loopform_continue_break_scan.hako

@@ -0,0 +1,40 @@
+// loopform_continue_break_scan.hako
+// 目的: Region+next_i 型の scan ループで continue + break 混在パターンをスモークする
+
+static box Main {
+  scan(s) {
+    local i = 0
+    local n = s.length()
+    // Case C-ish: constant-true + continue/backedge + break
+    loop(1 == 1) {
+      if i >= n { break }
+
+      // Region+next_i: この周回での「次の i」を 1 箇所に集約
+      local next_i = i
+      local ch = s.substring(i, i + 1)
+
+      if ch == " " {
+        next_i = i + 1
+        i = next_i
+        continue
+      }
+      if ch == "\t" {
+        next_i = i + 2
+        i = next_i
+        continue
+      }
+      // その他の文字で終了
+      break
+    }
+    return i
+  }
+
+  main(args) {
+    // 先頭に space + tab を並べたケースで scan が 3 を返すことを想定
+    local s = " \tX"
+    local r = Main.scan(s)
+    print(r)
+    return 0
+  }
+}
+

apps/tests/loopform_nested_region.hako

@@ -0,0 +1,39 @@
+// loopform_nested_region.hako
+// 目的: 外側 Loop + 内側 Region 型 Loop のネストをスモークする
+
+static box Main {
+  find_first_non_space(lines) {
+    local line_idx = 0
+    local n = lines.size()
+    // outer loop: 行単位
+    loop(line_idx < n) {
+      local line = lines.get(line_idx)
+      local col = 0
+
+      // inner loop: 行内の先頭非空白を探す（Region+next_col 風）
+      loop(1 == 1) {
+        if col >= line.length() { break }
+        local ch = line.substring(col, col + 1)
+        if ch == " " {
+          col = col + 1
+          continue
+        }
+        break
+      }
+
+      if col < line.length() { return line_idx }
+      line_idx = line_idx + 1
+    }
+    return -1
+  }
+
+  main(args) {
+    local lines = new ArrayBox()
+    lines.push("   ")
+    lines.push(" xx")
+    local r = Main.find_first_non_space(lines)
+    print(r)
+    return 0
+  }
+}
+

apps/tests/minimal_ssa_bug.hako

@@ -0,0 +1,20 @@
+// Minimal SSA Bug Reproduction
+// Pattern: Conditional reassignment with immediate usage
+// Expected: SSA violation "use of undefined value"
+
+static box MinimalSSABug {
+  main(args) {
+    local x = "test"
+
+    // Problem pattern: reassign + immediate use in nested condition
+    if x.length() > 0 {
+      x = x.substring(0, 2)            // reassign x
+      if x.length() > 0 && x.substring(0, 1) == "t" {  // immediate use
+        x = x.substring(1, 2)          // another reassign
+        print(x)
+      }
+    }
+
+    return 0
+  }
+}

apps/tests/minimal_ssa_bug_loop.hako

@@ -0,0 +1,28 @@
+// Minimal SSA Bug Reproduction with Loop
+// Pattern: Conditional reassignment inside loop with immediate usage
+// Expected: SSA violation "use of undefined value"
+
+static box MinimalSSABugLoop {
+  main(args) {
+    local src = "using foo\nusing bar\n"
+    local i = 0
+    local n = src.length()
+
+    loop(i < n) {
+      local line = src.substring(i, i + 10)
+
+      // Problem pattern: nested conditions with reassignment + immediate use
+      if line.length() > 0 {
+        line = line.substring(0, 5)     // reassign line
+        if line.length() > 0 && line.substring(0, 1) == "u" {  // immediate use
+          line = line.substring(1, 5)   // another reassign
+          print(line)
+        }
+      }
+
+      i = i + 10
+    }
+
+    return 0
+  }
+}

apps/tests/minimal_ssa_skip_ws.hako

@@ -0,0 +1,24 @@
+// minimal_ssa_skip_ws.hako — LoopForm exit-PHI regression canary
+// 目的: loop(1 == 1) + break だけで exit PHI が壊れる既知事例を最小化
+
+static box Main {
+  skip(s) {
+    local i = 0
+    local n = s.length()
+    // 本来は loop(i < n) だが、ワークアラウンドとして loop(1 == 1) にしていた経路で
+    // exit PHI が崩れて ValueId 未定義になる不具合を再現する。
+    loop(1 == 1) {
+      if i >= n { break }
+      local ch = s.substring(i, i + 1)
+      if ch == " " { i = i + 1 } else { break }
+    }
+    return i
+  }
+
+  main(args) {
+    local s = "   abc"
+    local r = Main.skip(s)
+    print(r)
+    return 0
+  }
+}

apps/tests/stage1_using_minimal.hako

@@ -0,0 +1 @@
+using "foo/bar.hako" as Foo

apps/tests/stage1_using_namespace.hako

@@ -0,0 +1,9 @@
+using std.collections
+using "path/to/module.hako" as MyModule
+
+static box Main {
+  main(args) {
+    print("Hello")
+    return 0
+  }
+}

apps/tests/test_else_if_scope.hako

@@ -0,0 +1,23 @@
+static box Main {
+  main(args) {
+    local x = args
+    if x == null {
+      x = new ArrayBox()
+      x.push("default")
+      if x.length() > 0 {
+        local status = "initialized"
+        if x.get(0) != null {
+          status = "has_value: " + x.length()
+        }
+        print("Status: " + status)
+      }
+    } else if x.length() >= 0 {
+      print("Length: " + ("" + x.length()))
+    }
+    if x == null {
+      return 1
+    }
+    print("Final length: " + ("" + x.length()))
+    return 0
+  }
+}

apps/tests/test_loop_conditional_assign.hako

@@ -0,0 +1,15 @@
+static box Main {
+  main(args) {
+    local str = "foo/bar/baz"
+    local idx = -1
+    local t = 0
+    loop(t < str.length()) {
+      if str.substring(t, t+1) == "/" {
+        idx = t
+      }
+      t = t + 1
+    }
+    print("Last slash at: " + ("" + idx))
+    return 0
+  }
+}

apps/tests/test_nested_scope_phi.hako

@@ -0,0 +1,29 @@
+static box Main {
+  main(args) {
+    local src = "using foo\nusing bar"
+    local n = src.length()
+    local i = 0
+    loop(i < n) {
+      local j = i
+      loop(j < n && src.substring(j, j+1) != "\n") { j = j + 1 }
+      local line = src.substring(i, j)
+
+      if line.length() > 0 {
+        local p = line
+        local idx = -1
+        local t = 0
+        loop(t < p.length()) {
+          if p.substring(t,t+1) == " " { idx = t }
+          t = t + 1
+        }
+        if idx >= 0 {
+          p = p.substring(0, idx)
+        }
+        print("Processed: " + p)
+      }
+
+      i = j + 1
+    }
+    return 0
+  }
+}

apps/tests/test_string_concat_phi.hako

@@ -0,0 +1,28 @@
+static box TestBox {
+  get_value() {
+    return "test_result"
+  }
+}
+
+static box Main {
+  main(args) {
+    local prog = TestBox.get_value()
+    {
+      local status = "null"
+      if prog != null { status = "non-null" }
+      print("Debug: " + status)
+    }
+    if prog == null {
+      print("Error: prog is null")
+      return null
+    }
+    local ps = "" + prog
+    print("Stringified length: " + ("" + ps.length()))
+    if ps.indexOf("test") < 0 || ps.indexOf("result") < 0 {
+      print("Error: unexpected output")
+      return null
+    }
+    print("Success")
+    return ps
+  }
+}