feat(joinir): Phase 224 - DigitPosPromoter for A-4 pattern (core complete)

- New DigitPosPromoter Box for cascading indexOf pattern detection
- Two-tier promotion strategy: A-3 Trim → A-4 DigitPos fallback
- Unit tests 6/6 PASS (comparison operators, cascading dependency)
- Promotion verified: digit_pos → is_digit_pos carrier

⚠️ Lowerer integration gap: lower_loop_with_break_minimal doesn't
recognize promoted variables yet (Phase 224-continuation)

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

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

src/mir/loop_pattern_detection/mod.rs

@@ -781,6 +781,9 @@ pub mod loop_body_carrier_promoter;
 // Phase 223-3: LoopBodyLocal Condition Promotion (for Pattern4)
 pub mod loop_body_cond_promoter;
 
+// Phase 224: A-4 DigitPos Pattern Promotion
+pub mod loop_body_digitpos_promoter;
+
 // Phase 171-C-5: Trim Pattern Helper
 pub mod trim_loop_helper;
 pub use trim_loop_helper::TrimLoopHelper;