feat(joinir): Phase 231 - ExprLowerer/ScopeManager pilot implementation

Pilot implementation of unified expression lowering for Pattern2 break conditions:

New files:
- scope_manager.rs (280 lines) - ScopeManager trait + Pattern2ScopeManager
- expr_lowerer.rs (455 lines) - ExprLowerer with Condition context support

Features:
- Unified variable lookup across ConditionEnv/LoopBodyLocalEnv/CapturedEnv/CarrierInfo
- Pre-validation of condition AST before lowering
- Fail-safe design with fallback to legacy path
- 8 new unit tests (all pass)

Integration:
- Pattern2 break condition uses ExprLowerer for pre-validation
- Existing proven lowering path preserved
- Zero impact on existing functionality (890/897 tests pass)

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

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

src/mir/builder/control_flow/joinir/patterns/pattern2_with_break.rs

@@ -529,9 +529,44 @@ impl MirBuilder {
             }
         }
 
+        // Phase 231: ExprLowerer pilot - pre-validate break condition
+        // This is a VALIDATION-ONLY step. We check if ExprLowerer can handle the condition,
+        // but still use the existing proven lowering path. Future phases will replace actual lowering.
+        {
+            use crate::mir::join_ir::lowering::scope_manager::{Pattern2ScopeManager, ScopeManager};
+            use crate::mir::join_ir::lowering::expr_lowerer::{ExprLowerer, ExprContext, ExprLoweringError};
+
+            let scope_manager = Pattern2ScopeManager {
+                condition_env: &env,
+                loop_body_local_env: Some(&body_local_env),
+                captured_env: Some(&captured_env),
+                carrier_info: &carrier_info,
+            };
+
+            // Try ExprLowerer validation (doesn't affect actual lowering yet)
+            // Phase 231: This is data-gathering only - we want to see which patterns work
+            match ExprLowerer::new(&scope_manager, ExprContext::Condition, self)
+                .with_debug(debug)
+                .lower(&effective_break_condition)
+            {
+                Ok(_value_id) => {
+                    eprintln!("[pattern2/phase231] ✓ ExprLowerer successfully validated break condition");
+                }
+                Err(ExprLoweringError::UnsupportedNode(msg)) => {
+                    eprintln!("[pattern2/phase231] ℹ ExprLowerer fallback (unsupported): {}", msg);
+                    // This is expected for complex patterns - not an error
+                }
+                Err(e) => {
+                    eprintln!("[pattern2/phase231] ⚠ ExprLowerer validation error: {}", e);
+                    // Unexpected error - log but don't fail (legacy path will handle it)
+                }
+            }
+        }
+
         // Phase 169 / Phase 171-fix / Phase 172-3 / Phase 170-B: Call Pattern 2 lowerer with break_condition
         // Phase 33-14: Now returns (JoinModule, JoinFragmentMeta) for expr_result + carrier separation
         // Phase 176-3: Multi-carrier support - pass carrier_info and carrier_updates
+        // Phase 231: ExprLowerer validated above, but we still use proven legacy lowering
         eprintln!(
             "[pattern2/before_lowerer] About to call lower_loop_with_break_minimal with carrier_info.loop_var_name='{}'",
             carrier_info.loop_var_name