Add experimental JoinIR runner and tests

docs/reference/environment-variables.md

@@ -106,6 +106,7 @@ NYASH_PARSER_STAGE3=1 NYASH_ENABLE_USING=1 \
 | `NYASH_STAGE1_SCAN_GE=1` | OFF | Compare Ge命令スキャン | Phase 25.x |
 | `NYASH_TO_I64_DEBUG=1` | OFF | to_i64変換デバッグ | Phase 25.x |
 | `NYASH_FUNCSCANNER_DEBUG=1` | OFF | FuncScanner詳細ログ | Phase 25.3 |
+| `NYASH_JOINIR_EXPERIMENT=1` | OFF | JoinIR実験モード（MIR→JoinIR変換テストを有効化） | Phase 26-H/27 |
 
 ### 使用例
 
@@ -120,6 +121,9 @@ NYASH_VM_DUMP_MIR=1 ./target/release/hakorune program.hako
 
 # Stage-1 CLI + MIRダンプ
 NYASH_STAGE1_MIR_DUMP=1 cargo test mir_stage1_cli_emit_program_min
+
+# JoinIR実験（テスト限定）
+NYASH_JOINIR_EXPERIMENT=1 cargo test --release mir_joinir_funcscanner_trim_auto_lowering -- --ignored
 ```
 
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