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_vm_bridge/mod.rs

@@ -0,0 +1,73 @@
+//! Phase 27-shortterm S-4: JoinIR → Rust VM Bridge
+//!
+//! 目的: JoinIR（正規化された IR）を Rust VM で実行するブリッジ層
+//!
+//! ## Architecture
+//! ```text
+//! JoinIR (normalized) → MirModule → Rust VM → Result
+//!       ↑                 ↑          ↑
+//!   PHI bugs         VM input    Execution
+//!   eliminated         format    (GC, plugins)
+//! ```
+//!
+//! ## Design Principles
+//! - JoinIR の正規化構造を保持したまま VM に渡す
+//! - マッピングだけで済ませる（JoinIR でやった正規化は消えない）
+//! - VM の機能（GC、プラグイン、エラーハンドリング）を活用
+//!
+//! ## Minimal Instruction Set (S-4.3)
+//! - **Compute**: Const, BinOp, Compare
+//! - **BoxCall**: StringBox メソッド呼び出し
+//! - **Call/Jump/Ret**: 制御フロー
+//!
+//! Phase 27-shortterm scope: skip_ws で green 化できれば成功
+
+use crate::backend::VMError;
+use crate::mir::join_ir::JoinFuncId;
+
+#[macro_use]
+mod logging {
+    macro_rules! debug_log {
+        ($($arg:tt)*) => {
+            if crate::config::env::joinir_vm_bridge_debug() {
+                eprintln!($($arg)*);
+            }
+        };
+    }
+}
+
+mod convert;
+mod meta;
+mod runner;
+
+#[cfg(test)]
+mod tests;
+
+pub(crate) use convert::{convert_join_function_to_mir, convert_joinir_to_mir};
+pub use meta::convert_join_module_to_mir_with_meta;
+pub use runner::run_joinir_via_vm;
+
+/// Phase 27-shortterm S-4 エラー型
+#[derive(Debug, Clone)]
+pub struct JoinIrVmBridgeError {
+    pub message: String,
+}
+
+impl JoinIrVmBridgeError {
+    pub fn new(msg: impl Into<String>) -> Self {
+        Self {
+            message: msg.into(),
+        }
+    }
+}
+
+impl From<VMError> for JoinIrVmBridgeError {
+    fn from(err: VMError) -> Self {
+        JoinIrVmBridgeError::new(format!("VM error: {:?}", err))
+    }
+}
+
+/// JoinFuncId から MIR 用の関数名を生成
+pub(crate) fn join_func_name(id: JoinFuncId) -> String {
+    format!("join_func_{}", id.0)
+}