feat(joinir): Phase 89 P1 - ContinueReturn lowering implementation

## ContinueReturn 独立箱
- continue_return_pattern.rs (457行)
- Continue + Early Return の JoinIR lowering
- StepCalculator 再利用（3箇所）
- Fail-Fast 原則徹底（5箇所検証）

## Dispatcher 配線
- mod.rs: LoopPattern::ContinueReturn 対応
- allowlist: pattern_continue_return_minimal 追加

## Fixture & Tests
- fixtures.rs: build_pattern_continue_return_min
- shapes.rs: +2 tests (vm_bridge + 期待値 n=10→acc=4)

Impact:
- normalized_dev: 61 passed (+2)
- lib tests: 993 passed (回帰なし)
- 箱化原則：単一責任・境界明確・再利用性高

Next: Phase 90 - _parse_string 合成 fixture

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

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

tests/normalized_joinir_min.rs

@@ -18,6 +18,7 @@ use nyash_rust::mir::join_ir::normalized::fixtures::{
     build_pattern3_if_sum_multi_min_structured_for_normalized_dev,
     build_pattern3_json_if_sum_min_structured_for_normalized_dev,
     build_pattern4_continue_min_structured_for_normalized_dev,
+    build_pattern_continue_return_min_structured_for_normalized_dev,
     build_selfhost_args_parse_p2_structured_for_normalized_dev,
     build_selfhost_if_sum_p3_ext_structured_for_normalized_dev,
     build_selfhost_if_sum_p3_structured_for_normalized_dev,