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/mod.rs

@@ -185,6 +185,15 @@ pub enum JoinInst {
     /// ルート関数 or 上位への戻り
     Ret { value: Option<VarId> },
 
+    /// Phase 33: If/Else の単純な値選択
+    /// cond が true なら then_val、false なら else_val を dst に代入
+    Select {
+        dst: VarId,
+        cond: VarId,
+        then_val: VarId,
+        else_val: VarId,
+    },
+
     /// それ以外の演算は、現行 MIR の算術/比較/boxcall を再利用
     Compute(MirLikeInst),
 }