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/frontend/ast_lowerer/mod.rs

@@ -89,6 +89,11 @@ fn resolve_function_route(func_name: &str) -> Result<FunctionRoute, String> {
             "jsonparser_parse_object_continue_skip_ws",
             FunctionRoute::LoopFrontend,
         ),
+        // Phase 88: JsonParser _unescape_string core (step2 + continue) minimal fixture
+        (
+            "jsonparser_unescape_string_step2_min",
+            FunctionRoute::LoopFrontend,
+        ),
     ];
 
     if let Some((_, route)) = TABLE.iter().find(|(name, _)| *name == func_name) {