refactor(joinir): Phase 183-2 Consolidate CarrierInfo initialization

Makes CarrierInfo::from_variable_map() the primary initialization method.
Common pattern initializer now delegates to this centralized logic.

## Changes

1. **Primary Method: CarrierInfo::from_variable_map()**:
   - Now the single source of truth for CarrierInfo construction
   - Used by both MIR and JoinIR contexts
   - Documented as primary initialization method (Phase 183-2)

2. **CommonPatternInitializer Refactoring**:
   - Converted to thin wrapper around `CarrierInfo::from_variable_map()`
   - Delegates carrier collection to primary method
   - Only adds pattern-specific exclusion filtering
   - Reduced code duplication (~30 lines removed)

3. **Documentation Updates**:
   - `carrier_info.rs`: Added Phase 183-2 section explaining primary role
   - `common_init.rs`: Documented delegation strategy
   - Clear separation of concerns between modules

4. **Removed Duplicate Logic**:
   - Eliminated manual carrier collection in `common_init.rs`
   - Removed `CarrierVar` import (no longer directly constructed)
   - Unified sorting and validation in one place

## Benefits

- **Single source of truth**: CarrierInfo construction logic in one module
- **Consistency**: Same initialization algorithm across MIR/JoinIR
- **Maintainability**: Changes to carrier logic only needed once
- **Testability**: Primary logic tested in carrier_info module

## Testing

✅ All carrier_info tests pass (7 tests)
✅ All pattern tests pass (124 tests)
✅ No behavioral changes

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

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

src/mir/builder/control_flow/joinir/patterns/common_init.rs

@@ -5,7 +5,7 @@
 //! ## Responsibility
 //!
 //! - Extract loop variable from condition AST
-//! - Build CarrierInfo from variable_map
+//! - Build CarrierInfo from variable_map (delegates to `CarrierInfo::from_variable_map`)
 //! - Support pattern-specific carrier exclusions (e.g., Pattern 2 excludes break-triggered vars)
 //!
 //! ## Usage
@@ -26,11 +26,16 @@
 //! - **Testability**: Can be tested independently
 //! - **Maintainability**: Changes to initialization only need to happen once
 //! - **Reduces duplication**: Eliminates 80 lines across Pattern 1-4
+//!
+//! # Phase 183-2: Delegation to CarrierInfo
+//!
+//! This module is now a thin wrapper around `CarrierInfo::from_variable_map()`.
+//! The primary logic lives in `carrier_info.rs` for consistency across MIR and JoinIR contexts.
 
 use crate::mir::builder::MirBuilder;
 use crate::mir::ValueId;
 use crate::ast::ASTNode;
-use crate::mir::join_ir::lowering::carrier_info::{CarrierInfo, CarrierVar};
+use crate::mir::join_ir::lowering::carrier_info::CarrierInfo;
 use std::collections::BTreeMap;
 
 pub struct CommonPatternInitializer;
@@ -89,30 +94,23 @@ impl CommonPatternInitializer {
                 )
             })?;
 
-        // Step 2: Collect carriers from variable_map
-        let mut carriers = Vec::new();
-        for (var_name, &var_id) in variable_map {
-            if var_name != &loop_var_name {
-                // Check exclusions (Pattern 2 specific, or other pattern-specific needs)
-                if let Some(excluded) = exclude_carriers {
-                    if excluded.contains(&var_name.as_str()) {
-                        continue;
-                    }
-                }
-                carriers.push(CarrierVar {
-                    name: var_name.clone(),
-                    host_id: var_id,
-                    join_id: None, // Phase 177-STRUCT-1
-                });
-            }
+        // Phase 183-2: Delegate to CarrierInfo::from_variable_map for consistency
+        // Convert BTreeMap to HashMap for compatibility
+        let mut variable_map_hashmap = std::collections::HashMap::new();
+        for (k, v) in variable_map {
+            variable_map_hashmap.insert(k.clone(), *v);
         }
 
-        let carrier_info = CarrierInfo {
-            loop_var_name: loop_var_name.clone(),
-            loop_var_id,
-            carriers,
-            trim_helper: None, // Phase 171-C-5: No Trim pattern by default
-        };
+        // Step 2: Use CarrierInfo::from_variable_map as primary initialization method
+        let mut carrier_info = CarrierInfo::from_variable_map(
+            loop_var_name.clone(),
+            &variable_map_hashmap,
+        )?;
+
+        // Step 3: Apply exclusions if provided (Pattern 2 specific)
+        if let Some(excluded) = exclude_carriers {
+            carrier_info.carriers.retain(|c| !excluded.contains(&c.name.as_str()));
+        }
 
         Ok((loop_var_name, loop_var_id, carrier_info))
     }

src/mir/join_ir/lowering/carrier_info.rs

@@ -5,6 +5,15 @@
 //! without hardcoded variable names or ValueIds.
 //!
 //! Phase 193-2: Enhanced builder methods for flexible construction
+//!
+//! # Phase 183-2: Primary CarrierInfo Construction
+//!
+//! This module is the single source of truth for CarrierInfo initialization.
+//! Both MIR and JoinIR contexts use `CarrierInfo::from_variable_map()` as the
+//! primary construction method.
+//!
+//! - MIR context: `common_init.rs` delegates to this module
+//! - JoinIR context: Uses `from_variable_map()` directly
 
 use crate::mir::ValueId;
 use std::collections::HashMap;