refactor(joinir): Phase 183-1 Unify pattern detection in loop_pattern_detection

Consolidates duplicate pattern detection logic across two routing layers.

## Changes

1. **Unified Detection Documentation**:
   - Added Phase 183 comments to `loop_pattern_detection::classify()`
   - Documented that this is the single source of truth for pattern classification
   - Both routers now reference this centralized function

2. **Router Documentation Updates**:
   - `patterns/router.rs`: Added Phase 183 comments explaining structure-based routing
   - `loop_pattern_router.rs`: Added unified detection section
   - Both routers now explicitly reference shared detection logic

3. **Improved Debug Output**:
   - Added `pattern_kind` to debug message in `route_loop_pattern()`
   - Helps diagnose pattern matching failures

## Benefits

- **Single source of truth**: Pattern classification logic in one place
- **Consistency**: Both routers use same detection algorithm
- **Maintainability**: Changes to classification rules only needed once
- **Documentation**: Clear references between routers and detection module

## Testing

✅ All loop_pattern_detection tests pass
✅ Pattern 2 tests pass
✅ No behavioral changes, pure documentation/organization refactoring

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

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

src/mir/loop_pattern_detection/mod.rs

@@ -259,6 +259,12 @@ pub fn extract_features(loop_form: &LoopForm, scope: Option<&LoopScopeShape>) ->
 ///
 /// # Returns
 /// * `LoopPatternKind` - Classified pattern
+///
+/// # Phase 183: Unified Detection
+///
+/// This is the single source of truth for pattern classification.
+/// Both routers (`router.rs` and `loop_pattern_router.rs`) use this
+/// function to avoid duplicate detection logic.
 pub fn classify(features: &LoopFeatures) -> LoopPatternKind {
     // Pattern 4: Continue (highest priority)
     if features.has_continue {