feat(joinir): Phase 170-C-2b LoopUpdateSummary wiring into shape detection

Wire LoopUpdateSummary box into real code paths:

- Add `update_summary: Option<LoopUpdateSummary>` field to LoopFeatures
- Add `detect_with_updates()` method to CaseALoweringShape
  - Uses UpdateKind (CounterLike/AccumulationLike) for classification
  - Single CounterLike carrier → StringExamination
  - AccumulationLike present → ArrayAccumulation or IterationWithAccumulation
- Update loop_to_join.rs to use detect_with_updates()
- Update deprecated detect() to use detect_with_updates() internally
- Set update_summary: None in AST-based feature extraction

Carrier name heuristics now encapsulated in LoopUpdateSummary box.
No regression: 15/16 loop smoke tests pass (1 failure is pre-existing).

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

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

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

@@ -90,6 +90,8 @@ pub fn extract_features(
         carrier_count,
         break_count: if has_break { 1 } else { 0 },
         continue_count: if has_continue { 1 } else { 0 },
+        // Phase 170-C-2b: AST-based extraction doesn't have carrier names yet
+        update_summary: None,
     }
 }