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/join_ir/lowering/carrier_info.rs

@@ -18,6 +18,29 @@
 use crate::mir::ValueId;
 use std::collections::BTreeMap; // Phase 222.5-D: HashMap → BTreeMap for determinism
 
+/// Phase 224-D: Alias for promoted LoopBodyLocal in condition expressions
+///
+/// When a LoopBodyLocal variable is promoted to a carrier, the original variable
+/// name must still be resolvable in condition expressions for backward compatibility.
+///
+/// # Example
+///
+/// ```ignore
+/// // Original variable: "digit_pos"
+/// // Promoted carrier: "is_digit_pos"
+/// ConditionAlias {
+///     old_name: "digit_pos".to_string(),
+///     carrier_name: "is_digit_pos".to_string(),
+/// }
+/// ```
+#[derive(Debug, Clone)]
+pub struct ConditionAlias {
+    /// Original variable name (e.g., "digit_pos")
+    pub old_name: String,
+    /// Promoted carrier name (e.g., "is_digit_pos")
+    pub carrier_name: String,
+}
+
 /// Information about a single carrier variable
 #[derive(Debug, Clone)]
 pub struct CarrierVar {
@@ -52,6 +75,12 @@ pub struct CarrierInfo {
     /// during condition promotion (e.g., DigitPosPromoter). The lowerer should skip
     /// LoopBodyLocal checks for these variables.
     pub promoted_loopbodylocals: Vec<String>,
+    /// Phase 224-D: Condition aliases for promoted LoopBodyLocal variables
+    ///
+    /// Maps old variable names to their promoted carrier names for condition resolution.
+    /// This allows break/continue conditions to reference promoted variables by their
+    /// original names (e.g., `if digit_pos < 0` still works after promotion to "is_digit_pos").
+    pub condition_aliases: Vec<ConditionAlias>,
 }
 
 impl CarrierInfo {
@@ -113,6 +142,7 @@ impl CarrierInfo {
             carriers,
             trim_helper: None, // Phase 171-C-5: No Trim pattern by default
             promoted_loopbodylocals: Vec::new(), // Phase 224: No promoted variables by default
+            condition_aliases: Vec::new(), // Phase 224-D: No aliases by default
         })
     }
 
@@ -171,6 +201,7 @@ impl CarrierInfo {
             carriers,
             trim_helper: None, // Phase 171-C-5: No Trim pattern by default
             promoted_loopbodylocals: Vec::new(), // Phase 224: No promoted variables by default
+            condition_aliases: Vec::new(), // Phase 224-D: No aliases by default
         })
     }
 
@@ -198,6 +229,7 @@ impl CarrierInfo {
             carriers,
             trim_helper: None, // Phase 171-C-5: No Trim pattern by default
             promoted_loopbodylocals: Vec::new(), // Phase 224: No promoted variables by default
+            condition_aliases: Vec::new(), // Phase 224-D: No aliases by default
         }
     }
 
@@ -258,6 +290,13 @@ impl CarrierInfo {
                 self.promoted_loopbodylocals.push(promoted_var.clone());
             }
         }
+
+        // Phase 224-D: Merge condition_aliases (deduplicate by old_name)
+        for alias in &other.condition_aliases {
+            if !self.condition_aliases.iter().any(|a| a.old_name == alias.old_name) {
+                self.condition_aliases.push(alias.clone());
+            }
+        }
     }
 
     /// Phase 171-C-5: Get Trim pattern helper