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/refactoring/candidates_phase15.md

@@ -18,7 +18,7 @@ High‑Value Candidates
 
 - VM dispatch integration
   - Gradually route the big `match` in VM through `backend/dispatch.rs` (already scaffolded). Start with side‑effect‑free ops (Const/Compare/TypeOp) to de‑risk.
-  - Add opt‑in flag `NYASH_VM_USE_DISPATCH=1` to gate behavior during the transition.
+  - (撤去済み) `NYASH_VM_USE_DISPATCH=1` でのディスパッチ経路スイッチ案は廃止。
 
 - Env access centralization
   - Replace scattered `std::env::var("NYASH_*")` with `config::env` getters in hot paths (VM tracing, GC barriers, resolver toggles).
@@ -42,4 +42,3 @@ Suggested Sequencing
 Notes
 - Keep JIT/Cranelift untouched in this phase to avoid drift from the mainline policy.
 - Prefer file‑level docs on new modules to guide incremental migration.
-