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/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;