refactor(joinir): Phase 170-A Step 1.5 - LoopFeatures-based detection API

Clarify design principle: CaseALoweringShape does NOT look at function names.
Input must be LoopFeatures/LoopPatternKind only (structure-based detection).

Changes:
- Add detect_from_features(LoopFeatures, carrier_count, has_progress_carrier)
- Deprecate detect(LoopScopeShape) with backward-compat wrapper
- Case-A now rejects loops with `has_continue` (only break is allowed)
- Document Phase 170-B future work (loop body AST analysis)

Design Principle:
> "CaseALoweringShape は 関数名を見ない。LoopFeatures/LoopPatternKind だけを入力にする"

This ensures generic routing works for ANY structurally matching loop.

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

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

src/mir/join_ir/lowering/loop_scope_shape/case_a_lowering_shape.rs

@@ -82,32 +82,48 @@ pub enum CaseALoweringShape {
 }
 
 impl CaseALoweringShape {
-    /// Detect lowering shape from LoopScopeShape
+    /// Detect lowering shape from LoopFeatures
     ///
-    /// Phase 170-A: Heuristic-based shape detection
+    /// # Phase 170-A Design Principle
     ///
-    /// Returns NotCaseA if structural requirements not met.
-    /// Returns one of the above if structural analysis suggests a pattern.
+    /// **CaseALoweringShape does NOT look at function names.**
+    /// Input: LoopFeatures / LoopPatternKind only (structure-based detection).
+    ///
+    /// This is the core architectural invariant that enables generic routing.
     ///
     /// # Heuristics
     /// - Single carrier (1) → likely StringExamination or ArrayAccumulation
     /// - Multiple carriers (2+) → likely IterationWithAccumulation
-    /// - Pinned count → disambiguation hint
+    /// - has_break/has_continue → affects Case-A eligibility
     ///
-    /// Note: More sophisticated pattern matching (analyzing loop body structure)
-    /// is deferred to Phase 170-B.
-    pub fn detect(scope: &super::shape::LoopScopeShape) -> Self {
-        use super::shape::LoopScopeShape;
+    /// # Phase 170-B Future Work
+    /// - Analyze loop body AST for more precise classification
+    /// - Distinguish StringExamination vs ArrayAccumulation
+    ///
+    /// # Arguments
+    /// * `features` - LoopFeatures (structure-based, name-agnostic)
+    /// * `carrier_count` - Number of carrier variables from LoopScopeShape
+    /// * `has_progress_carrier` - Whether progress carrier exists
+    pub fn detect_from_features(
+        features: &crate::mir::loop_pattern_detection::LoopFeatures,
+        carrier_count: usize,
+        has_progress_carrier: bool,
+    ) -> Self {
+        // Case-A requirement: must have a progress carrier
+        if !has_progress_carrier {
+            return CaseALoweringShape::NotCaseA;
+        }
 
-        // Check if structurally Case-A
-        if scope.progress_carrier.is_none() {
+        // Case-A requirement: no complex control flow (continue)
+        // Note: break is allowed (StringExamination patterns use break)
+        if features.has_continue {
             return CaseALoweringShape::NotCaseA;
         }
 
         // Phase 170-A: Simple heuristic based on carrier count
-        match scope.carriers.len() {
+        match carrier_count {
             0 => {
-                // This shouldn't happen if progress_carrier is Some, but be safe
+                // This shouldn't happen if has_progress_carrier is true, but be safe
                 CaseALoweringShape::NotCaseA
             }
             1 => {
@@ -124,6 +140,33 @@ impl CaseALoweringShape {
         }
     }
 
+    /// Legacy wrapper: Detect from LoopScopeShape (deprecated, use detect_from_features)
+    ///
+    /// Phase 170-A: Kept for backward compatibility during transition.
+    #[deprecated(
+        since = "Phase 170-A",
+        note = "Use detect_from_features() with LoopFeatures instead"
+    )]
+    pub fn detect(scope: &super::shape::LoopScopeShape) -> Self {
+        // Construct minimal LoopFeatures from LoopScopeShape
+        // Note: This loses some information (has_break, has_continue not available)
+        let has_progress_carrier = scope.progress_carrier.is_some();
+        let carrier_count = scope.carriers.len();
+
+        // Create stub features (Phase 170-B will use real LoopFeatures)
+        let stub_features = crate::mir::loop_pattern_detection::LoopFeatures {
+            has_break: false,       // Unknown from LoopScopeShape alone
+            has_continue: false,    // Unknown from LoopScopeShape alone
+            has_if: false,
+            has_if_else_phi: false,
+            carrier_count,
+            break_count: 0,
+            continue_count: 0,
+        };
+
+        Self::detect_from_features(&stub_features, carrier_count, has_progress_carrier)
+    }
+
     /// Is this a recognized lowering shape?
     pub fn is_recognized(&self) -> bool {
         !matches!(self, CaseALoweringShape::NotCaseA | CaseALoweringShape::Generic)