refactor(joinir): Document loop pattern detection architecture (Task 2)

Task 2: Loop pattern detection structure investigation
- Analyzed src/mir/loop_pattern_detection/ (~3k lines, not 26k as expected)
- Module is already well-structured with Phase 194+ architecture
- Legacy functions (Phase 188) are still actively used, not truly deprecated
- Added clarifying comment to prevent future confusion about "legacy" label

Decision: Skip reorganization - current structure is clean and functional.
No subdirectory needed for active production code.

src/mir/loop_pattern_detection/mod.rs

@@ -329,6 +329,12 @@ pub fn classify_with_diagnosis(features: &LoopFeatures) -> (LoopPatternKind, Str
 // ============================================================================
 // Legacy Detection Functions (Phase 188)
 // ============================================================================
+//
+// NOTE (Phase 179): These functions are still actively used by pattern lowerers.
+// The term "legacy" refers to Phase 188 implementation style (name-based detection),
+// not deprecation status. Do NOT move to legacy/ subdirectory - still production code.
+//
+// Future work: Gradually migrate to Phase 194+ structure-based detection (extract_features/classify).
 
 // ============================================================================
 // Pattern 1: Simple While Loop