refactor(joinir): Split ast_lowerer and join_ir_vm_bridge into modules

ast_lowerer.rs → ast_lowerer/ (10 files):
- mod.rs: public surface + entry dispatch
- context.rs: ExtractCtx helpers
- expr.rs: expression-to-JoinIR extraction
- if_return.rs: simple if→Select lowering
- loop_patterns.rs: loop variants (simple/break/continue)
- read_quoted.rs: read_quoted_from lowering (Phase 45-46)
- nested_if.rs: NestedIfMerge lowering
- analysis.rs: loop if-var analysis + metadata helpers
- tests.rs: frontend lowering tests
- README.md: module documentation

join_ir_vm_bridge.rs → join_ir_vm_bridge/ (5 files):
- mod.rs: public surface + shared helpers
- convert.rs: JoinIR→MIR lowering
- runner.rs: VM execution entry (run_joinir_via_vm)
- meta.rs: experimental metadata-aware hooks
- tests.rs: bridge-specific unit tests
- README.md: module documentation

Benefits:
- Clear separation of concerns per pattern
- Easier navigation and maintenance
- Each file has single responsibility
- README documents module boundaries

Co-authored-by: ChatGPT <noreply@openai.com>

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

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

src/mir/join_ir/frontend/ast_lowerer/mod.rs

@@ -0,0 +1,137 @@
+//! AST/CFG → JoinIR Lowering
+//!
+//! このモジュールは AST/CFG ノードを JoinIR 命令に変換する。
+//!
+//! ## 責務
+//!
+//! - **If 文→Select/IfMerge 変換**: 条件分岐を JoinIR の継続渡しスタイルに変換
+//! - **Loop 文→loop_step/k_exit 変換**: ループを関数呼び出しと継続に正規化
+//! - **Break/Continue/Return→k_* 変換**: 制御フローを継続 ID として表現
+//!
+//! ## Phase 34-2 での実装スコープ
+//!
+//! 最初は `IfSelectTest.*` 相当の tiny ケースのみ対応：
+//! - Simple pattern: `if cond { return 1 } else { return 2 }`
+//!
+//! ## 設計原則
+//!
+//! - **JoinIR = PHI 生成器**: 既存 PHI の変換器にはしない（Phase 33-10 原則）
+//! - **段階的移行**: 既存 MIR Builder 経路は保持、新経路はデフォルト OFF
+//! - **A/B テスト可能**: 既存経路と新経路の両方で実行して比較検証
+
+pub(crate) use crate::mir::join_ir::{
+    BinOpKind, CompareOp, ConstValue, JoinFuncId, JoinFunction, JoinInst, JoinModule, MergePair,
+    VarId,
+};
+pub(crate) use std::collections::{BTreeMap, HashSet};
+
+mod analysis;
+mod context;
+mod expr;
+mod if_return;
+mod loop_patterns;
+mod nested_if;
+mod read_quoted;
+
+#[cfg(test)]
+mod tests;
+
+pub(crate) use context::ExtractCtx;
+
+/// AST/CFG → JoinIR 変換器
+///
+/// Phase 34-2: Program(JSON v0) から tiny IfSelect ケースを JoinIR に変換
+pub struct AstToJoinIrLowerer {
+    pub(crate) next_func_id: u32,
+    #[allow(dead_code)]
+    pub(crate) next_var_id: u32,
+}
+
+impl AstToJoinIrLowerer {
+    /// 新しい lowerer を作成
+    pub fn new() -> Self {
+        Self {
+            next_func_id: 0,
+            next_var_id: 0,
+        }
+    }
+
+    /// Program(JSON v0) → JoinModule
+    ///
+    /// Phase 34-2/34-3/34-4: simple/local/json_shape pattern に対応
+    /// Phase 34-5: extract_value 統一化（Int/Var/Method 構造まで）
+    ///
+    /// # Panics
+    ///
+    /// - パターンに合わない Program(JSON) が来た場合（Phase 34 は tiny テスト専用）
+    /// - ループ・複数変数・副作用付き if（Phase 34-6 以降で対応予定）
+    pub fn lower_program_json(&mut self, program_json: &serde_json::Value) -> JoinModule {
+        // 1. Program(JSON) から defs を取得
+        let defs = program_json["defs"]
+            .as_array()
+            .expect("Program(JSON v0) must have 'defs' array");
+
+        // 2. 最初の関数定義を取得
+        let func_def = defs
+            .get(0)
+            .expect("At least one function definition required");
+
+        let func_name = func_def["name"]
+            .as_str()
+            .expect("Function must have 'name'");
+
+        // 3. 関数名で分岐（Phase 34-2/34-3/34-4/34-5/34-7/34-8/41-4/45）
+        // test/local/_read_value_from_pair: If Return pattern
+        // simple: Loop pattern (Phase 34-7/34-8)
+        // parse_loop: Phase 41-4 NestedIfMerge pattern
+        // read_quoted_from: Phase 45 Guard if + Loop with break + accumulator
+        match func_name {
+            "test" | "local" | "_read_value_from_pair" => {
+                self.lower_if_return_pattern(program_json)
+            }
+            "simple" => {
+                // Phase 34-8: Loop パターンの詳細分析（break/continue 検出）
+                self.lower_loop_with_break_continue(program_json)
+            }
+            "parse_loop" => {
+                // Phase 41-4: NestedIfMerge pattern (dev flag gated)
+                // Guard: HAKO_JOINIR_NESTED_IF=1 を要求
+                if std::env::var("HAKO_JOINIR_NESTED_IF").ok().as_deref() == Some("1") {
+                    self.lower_nested_if_pattern(program_json)
+                } else {
+                    // Dev flag が OFF の場合は panic（旧ルートにフォールバックするため）
+                    panic!(
+                        "parse_loop NestedIfMerge requires HAKO_JOINIR_NESTED_IF=1. \
+                         Set env var to enable Phase 41-4 route."
+                    );
+                }
+            }
+            "read_quoted_from" => {
+                // Phase 45: read_quoted_from pattern (dev flag gated)
+                // Guard if + Loop with break + accumulator
+                if std::env::var("HAKO_JOINIR_READ_QUOTED").ok().as_deref() == Some("1") {
+                    self.lower_read_quoted_pattern(program_json)
+                } else {
+                    panic!(
+                        "read_quoted_from JoinIR requires HAKO_JOINIR_READ_QUOTED=1. \
+                         Set env var to enable Phase 45 route."
+                    );
+                }
+            }
+            _ => panic!("Unsupported function: {}", func_name),
+        }
+    }
+
+    /// 次の関数 ID を生成
+    pub(crate) fn next_func_id(&mut self) -> JoinFuncId {
+        let id = JoinFuncId::new(self.next_func_id);
+        self.next_func_id += 1;
+        id
+    }
+}
+
+impl Default for AstToJoinIrLowerer {
+    fn default() -> Self {
+        Self::new()
+    }
+}