feat(joinir): Phase 224-D - ConditionAlias for promoted variable resolution

- Add ConditionAlias type to CarrierInfo (old_name → carrier_name)
- Record aliases in DigitPosPromoter and TrimPatternInfo
- Resolve aliases in Pattern2 ConditionEnv building
- digit_pos now correctly resolves to is_digit_pos carrier

Fixes "Variable 'digit_pos' not bound in ConditionEnv" error.

🤖 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_carrier_promoter.rs

@@ -83,6 +83,12 @@ impl TrimPatternInfo {
         // Phase 171-C-5: Attach TrimLoopHelper for pattern-specific lowering logic
         carrier_info.trim_helper = Some(TrimLoopHelper::from_pattern_info(self));
 
+        // Phase 224-D: Record condition alias for promoted variable
+        carrier_info.condition_aliases.push(crate::mir::join_ir::lowering::carrier_info::ConditionAlias {
+            old_name: self.var_name.clone(),
+            carrier_name: self.carrier_name.clone(),
+        });
+
         carrier_info
     }
 }

src/mir/loop_pattern_detection/loop_body_digitpos_promoter.rs

@@ -180,17 +180,37 @@ impl DigitPosPromoter {
                 );
 
                 // Step 6: Build CarrierInfo
+                // For DigitPos pattern, we add a NEW carrier (not replace loop_var)
                 let carrier_name = format!("is_{}", var_in_cond);
-                let carrier_info = CarrierInfo::with_carriers(
-                    carrier_name.clone(),
-                    ValueId(0), // Placeholder (will be remapped)
-                    vec![],
+
+                use crate::mir::join_ir::lowering::carrier_info::CarrierVar;
+                let promoted_carrier = CarrierVar {
+                    name: carrier_name.clone(),
+                    host_id: ValueId(0), // Placeholder (will be remapped)
+                    join_id: None, // Will be allocated later
+                };
+
+                // Create CarrierInfo with a dummy loop_var_name (will be ignored during merge)
+                let mut carrier_info = CarrierInfo::with_carriers(
+                    "__dummy_loop_var__".to_string(), // Placeholder, not used
+                    ValueId(0), // Placeholder
+                    vec![promoted_carrier],
                 );
 
+                // Phase 224-D: Record condition alias for promoted variable
+                carrier_info.condition_aliases.push(crate::mir::join_ir::lowering::carrier_info::ConditionAlias {
+                    old_name: var_in_cond.clone(),
+                    carrier_name: carrier_name.clone(),
+                });
+
                 eprintln!(
                     "[digitpos_promoter] A-4 DigitPos pattern promoted: {} → {}",
                     var_in_cond, carrier_name
                 );
+                eprintln!(
+                    "[digitpos_promoter] Phase 224-D: Added condition alias '{}' → '{}'",
+                    var_in_cond, carrier_name
+                );
 
                 return DigitPosPromotionResult::Promoted {
                     carrier_info,