feat(joinir): Phase 88 - Pattern4 continue + variable step increment

Continue Pattern 拡張:
- then側の i=i+const 差分加算 + acc更新を許可
- continue_pattern.rs:193 で可変ステップ検出

Dev Router 許可:
- ast_lowerer/mod.rs:92 で normalized_dev feature時に新パターンを有効化

Fixtures & Tests:
- jsonparser_unescape_string_step2_min fixture追加（submodule）
- normalized_joinir_min.rs に shape テスト追加
- shapes.rs に expected shape 定義

Documentation:
- joinir-architecture-overview.md に Phase 88 到達点を追記

Impact:
- Pattern4 continue + 可変インクリメント（i+=1 or i+=2）対応
- _unescape_string 制御構造の土台確立
- normalized_dev tests PASS

Next: _unescape_string 残り複合ループ対応

🤖 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

@@ -796,6 +796,33 @@ pub fn build_jsonparser_parse_object_continue_skip_ws_structured_for_normalized_
     module
 }
 
+/// JsonParser _unescape_string の「i+=2 + continue」コアを Structured で組み立てるヘルパー（dev-only）。
+///
+/// 実ループ（`tools/hako_shared/json_parser.hako::_unescape_string`）から、
+/// 文字列処理を除いて制御構造（continue + 可変ステップ更新）だけを抽出した最小フィクスチャ。
+///
+/// Fixture: docs/private/roadmap2/phases/normalized_dev/fixtures/jsonparser_unescape_string_step2_min.program.json
+pub fn build_jsonparser_unescape_string_step2_min_structured_for_normalized_dev() -> JoinModule {
+    const FIXTURE: &str = include_str!(
+        "../../../../docs/private/roadmap2/phases/normalized_dev/fixtures/jsonparser_unescape_string_step2_min.program.json"
+    );
+
+    let program_json: serde_json::Value = serde_json::from_str(FIXTURE)
+        .expect("jsonparser_unescape_string_step2_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] jsonparser_unescape_string_step2_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::{
@@ -806,6 +833,7 @@ pub mod prelude {
         build_jsonparser_parse_object_continue_skip_ws_structured_for_normalized_dev,
         build_jsonparser_skip_ws_real_structured_for_normalized_dev,
         build_jsonparser_skip_ws_structured_for_normalized_dev,
+        build_jsonparser_unescape_string_step2_min_structured_for_normalized_dev,
         build_pattern2_break_fixture_structured, build_pattern2_minimal_structured,
         build_pattern3_if_sum_min_structured_for_normalized_dev,
         build_pattern3_if_sum_multi_min_structured_for_normalized_dev,