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/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;