Phase 33-2: JoinInst::Select implementation + minimal If JoinIR lowering

Implementation:
- Add JoinInst::Select variant to JoinIR schema
- Implement Select execution in JoinIR Runner (Bool/Int cond support)
- Add Select handling in JoinIR→MIR Bridge (4-block structure)
- Create test cases (joinir_if_select_simple/local.hako)
- Add dev toggle NYASH_JOINIR_IF_SELECT=1
- Create lowering infrastructure (if_select.rs, stub for Phase 33-3)

Tests:
- 3/3 unit tests pass (test_select_true/false/int_cond)
- Integration tests pass (RC: 0)
- A/B execution verified (existing if_phi vs JoinIR Select)

Files changed:
- New: apps/tests/joinir_if_select_{simple,local}.hako
- New: src/mir/join_ir/lowering/if_select.rs
- Modified: src/mir/join_ir/{mod,json,runner,vm_bridge}.rs
- Modified: src/config/env.rs (joinir_if_select_enabled)
- Modified: docs/reference/environment-variables.md

Phase 33-3 ready: MIR pattern recognition + auto-lowering pending

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/join_ir/lowering/if_select.rs

@@ -0,0 +1,46 @@
+//! Phase 33: If/Else の Select 命令への lowering
+//!
+//! 最小の if/else（副作用なし、単純な値選択）を JoinInst::Select に変換する。
+
+use crate::mir::join_ir::JoinInst;
+use crate::mir::{BasicBlockId, MirFunction};
+
+pub struct IfSelectLowerer {
+    debug: bool,
+}
+
+impl IfSelectLowerer {
+    pub fn new(debug: bool) -> Self {
+        Self { debug }
+    }
+
+    /// if/else が Select に lowering できるかチェック
+    pub fn can_lower_to_select(&self, _func: &MirFunction, _if_block_id: BasicBlockId) -> bool {
+        // パターン:
+        // 1. if_block に Branch がある
+        // 2. then/else ブロックが存在
+        // 3. merge ブロックに 1 つの PHI がある
+        // 4. PHI の incoming が then/else から来ている
+
+        // 実装: Phase 33-2 では保守的に false を返す（フォールバック）
+        // Phase 33-3 で実装予定
+        if self.debug {
+            eprintln!("[joinir/if_select] can_lower_to_select: Phase 33-2 stub (always false)");
+        }
+        false
+    }
+
+    /// if/else を Select に変換
+    pub fn lower_if_to_select(
+        &self,
+        _func: &MirFunction,
+        _if_block_id: BasicBlockId,
+    ) -> Option<JoinInst> {
+        // 実装: Phase 33-2 では None を返す（フォールバック）
+        // Phase 33-3 で実装予定
+        if self.debug {
+            eprintln!("[joinir/if_select] lower_if_to_select: Phase 33-2 stub (always None)");
+        }
+        None
+    }
+}

src/mir/join_ir/lowering/mod.rs

@@ -12,12 +12,14 @@
 //! - `funcscanner_trim.rs`: FuncScannerBox.trim/1 の trim lowering
 //! - `stage1_using_resolver.rs`: Stage1UsingResolverBox.resolve_for_source entries loop lowering（Phase 27.12）
 //! - `funcscanner_append_defs.rs`: FuncScannerBox._append_defs/2 の配列結合 lowering（Phase 27.14）
+//! - `if_select.rs`: Phase 33 If/Else → Select lowering
 
 pub mod common;
 pub mod exit_args_resolver;
 pub mod funcscanner_append_defs;
 pub mod funcscanner_trim;
 pub mod generic_case_a;
+pub mod if_select; // Phase 33
 pub mod loop_form_intake;
 pub mod loop_scope_shape;
 pub mod loop_to_join;