feat(joinir): Phase 188.3-P3.1,3.2 - AST extraction & validation helpers

Phase 3-1 完了: extract_inner_loop_ast() 実装
- 外側 loop body から内側 loop を抽出
- 正確に 1 つの inner loop を検証
- 0 個 or 2+ 個の場合は明示エラー (Fail-Fast)

Phase 3-2 完了: validate_strict_mode() プレースホルダー
- 現在は大半の検証を is_pattern6_lowerable() で実施
- 将来の strict mode チェック用の足場

ドキュメント追加:
- P1-INSTRUCTIONS.md: Phase 3-3 実装指示書 (4関数モデル詳細)
- README.md: max_loop_depth 定義の明確化
- 10-Now.md: Phase 188.3 方針の整合性維持

次タスク: Phase 3-3 (Continuation 生成) - P1-INSTRUCTIONS.md 参照

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

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

src/mir/builder/control_flow/joinir/patterns/pattern6_nested_minimal.rs

@@ -22,11 +22,74 @@
 //! - Generate k_inner_exit (bridges to outer continuation)
 //! - Wire continuations
 
+use crate::ast::ASTNode;
 use crate::mir::builder::control_flow::joinir::patterns::LoopPatternContext;
 use crate::mir::builder::MirBuilder;
 use crate::mir::loop_pattern_detection::LoopPatternKind;
 use crate::mir::ValueId;
 
+/// Phase 188.3: Validate strict mode constraints
+///
+/// Requirements (most already checked in is_pattern6_lowerable):
+/// - Inner loop has no break (checked in routing.rs)
+/// - Inner loop has no continue (checked in routing.rs)
+/// - Outer loop has no break (checked in routing.rs)
+/// - Outer loop has no continue (checked in routing.rs)
+///
+/// Additional checks (Phase 188.4+):
+/// - Outer variable write-back detection (TODO: add to is_pattern6_lowerable)
+///
+/// This function serves as final safety check before lowering.
+/// If validation fails → Fail-Fast (not silent fallback).
+fn validate_strict_mode(_inner_ast: &ASTNode, _ctx: &LoopPatternContext) -> Result<(), String> {
+    // Phase 188.3: Most validation already done in is_pattern6_lowerable()
+    // This is a placeholder for future strict mode checks
+
+    // Future Phase 188.4+ checks:
+    // - Detect outer variable write-back in inner loop body
+    // - Validate carrier compatibility
+    // - Check for unsupported AST patterns
+
+    Ok(())
+}
+
+/// Phase 188.3: Extract inner loop AST from outer loop body
+///
+/// Requirements:
+/// - Outer body must contain exactly 1 Loop node
+/// - 0 loops → Error (shouldn't happen after Pattern6 selection)
+/// - 2+ loops → Error (multiple inner loops not supported)
+///
+/// Returns reference to inner loop ASTNode
+fn extract_inner_loop_ast<'a>(ctx: &'a LoopPatternContext) -> Result<&'a ASTNode, String> {
+    // Find all Loop nodes in outer body
+    let mut inner_loop: Option<&ASTNode> = None;
+    let mut loop_count = 0;
+
+    for stmt in ctx.body.iter() {
+        if matches!(stmt, ASTNode::Loop { .. }) {
+            loop_count += 1;
+            if inner_loop.is_none() {
+                inner_loop = Some(stmt);
+            }
+        }
+    }
+
+    // Validate exactly 1 inner loop
+    match loop_count {
+        0 => Err(
+            "[Pattern6/extract] No inner loop found (should not happen after Pattern6 selection)"
+                .to_string(),
+        ),
+        1 => Ok(inner_loop.unwrap()), // Safe: loop_count == 1 guarantees Some
+        _ => Err(format!(
+            "[Pattern6/extract] Multiple inner loops ({}) not supported. \
+             Phase 188.3 supports exactly 1 inner loop only.",
+            loop_count
+        )),
+    }
+}
+
 /// Detect if this context can be lowered as Pattern6 (NestedLoopMinimal)
 ///
 /// Pattern selection happens in choose_pattern_kind() (SSOT).
@@ -40,9 +103,15 @@ pub(crate) fn can_lower(_builder: &MirBuilder, ctx: &LoopPatternContext) -> bool
 /// Phase 188.3: Full implementation with continuation generation
 pub(crate) fn lower(
     _builder: &mut MirBuilder,
-    _ctx: &LoopPatternContext,
+    ctx: &LoopPatternContext,
 ) -> Result<Option<ValueId>, String> {
-    // Phase 188.3 stub - full implementation in Phase 3-3
+    // Phase 3-1: Extract inner loop AST (validate exactly 1 inner loop)
+    let inner_ast = extract_inner_loop_ast(ctx)?;
+
+    // Phase 3-2: Validate strict mode constraints (Fail-Fast)
+    validate_strict_mode(inner_ast, ctx)?;
+
+    // Phase 3-3 stub - full implementation next
     // TODO: Implement continuation generation (outer_step, inner_step, k_inner_exit)
-    Err("[Pattern6] Nested loop lowering not yet implemented (Phase 188.3 stub)".to_string())
+    Err("[Pattern6] Nested loop lowering not yet implemented (Phase 3-3 pending)".to_string())
 }