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>

src/mir/join_ir/normalized/fixtures.rs

@@ -823,6 +823,30 @@ pub fn build_jsonparser_unescape_string_step2_min_structured_for_normalized_dev(
     module
 }
 
+/// Pattern Continue + Return minimal を Structured で組み立てるヘルパー
+///
+/// Fixture: docs/private/roadmap2/phases/normalized_dev/fixtures/pattern_continue_return_min.program.json
+pub fn build_pattern_continue_return_min_structured_for_normalized_dev() -> JoinModule {
+    const FIXTURE: &str = include_str!(
+        "../../../../docs/private/roadmap2/phases/normalized_dev/fixtures/pattern_continue_return_min.program.json"
+    );
+
+    let program_json: serde_json::Value =
+        serde_json::from_str(FIXTURE).expect("pattern_continue_return_min fixture should be valid JSON");
+
+    let mut lowerer = AstToJoinIrLowerer::new();
+    let module = lowerer.lower_program_json(&program_json);
+
+    if joinir_dev_enabled() && joinir_test_debug_enabled() {
+        eprintln!(
+            "[joinir/normalized-dev] pattern_continue_return_min structured module: {:#?}",
+            module
+        );
+    }
+
+    module
+}
+
 /// まとめて import したいとき用のプレリュード。
 pub mod prelude {
     pub use super::{
@@ -839,6 +863,7 @@ pub mod prelude {
         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_if_sum_p3_ext_structured_for_normalized_dev,
         build_selfhost_if_sum_p3_structured_for_normalized_dev,
         build_selfhost_token_scan_p2_accum_structured_for_normalized_dev,