feat(phase284): P1 Complete - Return in Loop with Block Remap Fix

## Summary

Completed Phase 284 P1: Enable return statements in Pattern4/5 loops via
JoinInst::Ret infrastructure (100% pre-existing, no new infrastructure needed).

**Critical Bug Fix**: Block ID remap priority
- Fixed: local_block_map must take precedence over skipped_entry_redirects
- Root cause: Function-local block IDs can collide with global remap entries
  (example: loop_step:bb4 vs k_exit:bb4 after merge allocation)
- Impact: Conditional Jump else branches were incorrectly redirected to exit
- Solution: Check local_block_map FIRST, then skipped_entry_redirects

## Implementation

### New Files
- `src/mir/join_ir/lowering/return_collector.rs` - Return detection SSOT (top-level only, P1 scope)
- `apps/tests/phase284_p1_return_in_loop_min.hako` - Test fixture (exit code 7)
- Smoke test scripts (VM/LLVM)

### Modified Files
- `loop_with_continue_minimal.rs`: Return condition check + Jump generation
- `pattern4_with_continue.rs`: K_RETURN registration in continuation_funcs
- `canonical_names.rs`: K_RETURN constant
- `instruction_rewriter.rs`: Fixed Branch remap priority (P1 fix)
- `terminator.rs`: Fixed Jump/Branch remap priority (P1 fix)
- `conversion_pipeline.rs`: Return normalization support

## Testing

✅ VM: exit=7 PASS
✅ LLVM: exit=7 PASS
✅ Baseline: 46 PASS, 1 FAIL (pre-existing emit issue)
✅ Zero regression

## Design Notes

- JoinInst::Ret infrastructure was 100% complete before P1
- Bridge automatically converts JoinInst::Ret → MIR Return terminator
- Pattern4/5 now properly merge k_return as non-skippable continuation
- Correct semantics: true condition → return, false → continue loop

## Next Phase (P2+)

- Refactor: Block remap SSOT (block_remapper.rs)
- Refactor: Return jump emitter extraction
- Scope: Nested if/loop returns, multiple returns
- Design: Standardize early exit pattern (return/break/continue as Jump with cond)

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

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/development/current/main/30-Backlog.md

@@ -21,6 +21,29 @@ Related:
     - P0（docs-only）で “return の意味” と “Ok(None)/Err” の境界を固定
     - P1+ で Rust/LLVM の実装を SSOT に収束（pattern側に例外実装を増やさない）
 
+- **Phase 285（planned, design-first）: Box lifecycle / weakref / finalization / GC SSOT**
+  - 目的: Box の生存期間（強参照/弱参照/解放/最終化）を SSOT として固定し、「実装が仕様」になっている箇所を潰す
+  - ねらい:
+    - VM 側の weakref/finalization を仕様化（テストで固定）
+    - LLVM harness 側の未対応/差分を “仕様として明文化” し、将来の実装計画を切る
+  - 入口: `docs/development/current/main/phases/phase-285/README.md`
+  - スコープ案:
+    - P0（docs-only）: 仕様SSOT + 用語 + ルート集合（roots）+ 禁止事項（finalizer内での再入など）を固定
+    - P1（investigation）: Rust VM 実装の棚卸し + LLVM harness の現状差分を記録
+    - P2（smoke）: weakref の最小 fixture/smoke を VM/（可能なら）LLVM で固定
+  - 参考（現状の入口候補）:
+    - weakref 表現: `src/value.rs`（`NyashValue::WeakBox`）
+    - finalization: `src/finalization.rs`
+
+- **Phase 286（planned, design-first）: JoinIR Line Absorption（JoinIR→CorePlan/Frag 収束）**
+  - 目的: 移行期間に残っている「2本の lowering（Plan line / JoinIR line）」を、構造で 1 本に収束させる
+  - ねらい: `return/break/continue` のような “大きな出口語彙” の実装場所が揺れない状態にする
+  - 入口: `docs/development/current/main/phases/phase-286/README.md`
+  - P0（docs-only）: `docs/development/current/main/phases/phase-286/P0-INSTRUCTIONS.md`
+  - SSOT:
+    - Plan/Frag: `compose::*` + `emit_frag()`（Phase 280/281）
+    - JoinIR line 共通入口: `src/mir/builder/control_flow/joinir/patterns/conversion_pipeline.rs`
+
 - （✅ done）**Phase 282**: Router shrinkage + detection SSOT + extractor refactor
   - 完了: `docs/development/current/main/phases/phase-282/README.md`