feat(joinir): Phase 227 - CarrierRole separation (LoopState vs ConditionOnly)

- Add CarrierRole enum to distinguish state carriers from condition-only carriers
- ConditionOnly carriers (is_digit_pos) skip exit PHI but keep header PHI
- Update all test struct literals with role field
- 877/884 tests PASS (7 pre-existing failures unrelated)

Remaining: ValueId(0) undefined - header PHI initialization for ConditionOnly

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

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

src/mir/loop_pattern_detection/loop_body_digitpos_promoter.rs

@@ -183,11 +183,12 @@ impl DigitPosPromoter {
                 // For DigitPos pattern, we add a NEW carrier (not replace loop_var)
                 let carrier_name = format!("is_{}", var_in_cond);
 
-                use crate::mir::join_ir::lowering::carrier_info::CarrierVar;
+                use crate::mir::join_ir::lowering::carrier_info::{CarrierVar, CarrierRole};
                 let promoted_carrier = CarrierVar {
                     name: carrier_name.clone(),
                     host_id: ValueId(0), // Placeholder (will be remapped)
                     join_id: None, // Will be allocated later
+                    role: CarrierRole::ConditionOnly, // Phase 227: DigitPos is condition-only
                 };
 
                 // Create CarrierInfo with a dummy loop_var_name (will be ignored during merge)