feat(phase32): L-2.1 Stage-1 UsingResolver JoinIR integration + cleanup

Phase 32 L-2.1 complete implementation:

1. Stage-1 UsingResolver main line JoinIR connection
   - CFG-based LoopForm construction for resolve_for_source/5
   - LoopToJoinLowerer integration with handwritten fallback
   - JSON snapshot tests 6/6 PASS

2. JoinIR/VM Bridge improvements
   - Simplified join_ir_vm_bridge.rs dispatch logic
   - Enhanced json.rs serialization
   - PHI core boxes cleanup (local_scope_inspector, loop_exit_liveness, loop_var_classifier)

3. Stage-1 CLI enhancements
   - Extended args.rs, groups.rs, mod.rs for new options
   - Improved stage1_bridge module (args, env, mod)
   - Updated stage1_cli.hako

4. MIR builder cleanup
   - Simplified if_form.rs control flow
   - Removed dead code from loop_builder.rs
   - Enhanced phi_merge.rs

5. Runner module updates
   - json_v0_bridge/lowering.rs improvements
   - dispatch.rs, selfhost.rs, modes/vm.rs cleanup

6. Documentation updates
   - CURRENT_TASK.md, AGENTS.md
   - Various docs/ updates

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/development/design/extern-vs-boxcall.md

@@ -32,7 +32,7 @@ Extern vs BoxCall — 分離方針とスロット/アリティ一覧（Phase 12
 - BoxCall: vtable（TypeRegistry のスロット）→ PIC（poly→mono）→ 汎用メソッド呼び。
   - STRICT: 未登録メソッドは型名・メソッド名・arity・known一覧を含めてエラー。
 - ExternCall: `extern_registry` で iface/method/arity を登録、任意で slot 経由のハンドラに集約。
-  - `NYASH_EXTERN_ROUTE_SLOTS=1` で name→slot 専用ハンドラへ（VM/JITの挙動安定）。
+  - name→slot 専用ハンドラは検討のみ。旧 `NYASH_EXTERN_ROUTE_SLOTS` は未使用につき撤去。
 
 TypeRegistryの代表スロット
 - InstanceBox: 1(getField), 2(setField), 3(has), 4(size)
@@ -50,7 +50,6 @@ Extern スロット（抜粋）
 環境変数
 - `NYASH_ABI_VTABLE`: VMのvtable経路有効化
 - `NYASH_ABI_STRICT`: STRICT診断を有効化
-- `NYASH_EXTERN_ROUTE_SLOTS`: Externをslot経路に統一
+- `NYASH_EXTERN_ROUTE_SLOTS`: （撤去済み）Externをslot経路に統一するトグル
 - `NYASH_JIT_HOST_BRIDGE`: JITのhost-bridge（by-slot経路）を有効化
-- `NYASH_VM_PIC_THRESHOLD`: PICモノ化しきい値（既定=8）
-
+- `NYASH_VM_PIC_THRESHOLD`: （撤去済み）PICモノ化しきい値トグル