feat(joinir): Phase 171-172 Issue 5 - ConditionEnvBuilder unified construction

Eliminates 40-50 lines of duplicated ConditionEnv + ConditionBinding construction in Pattern 2.

**Changes**:
- New: `condition_env_builder.rs` with factory methods
- `build_for_break_condition()`: Extract condition variables, allocate JoinIR ValueIds, create bindings
- `build_loop_param_only()`: Simple env with only loop parameter
- Updated Pattern 2 to use unified builder
- Includes 4 unit tests covering all usage scenarios

**Impact**:
- Pattern 2: 47 lines → 11 lines (36-line reduction, ~77%)
- Preserved allocator closure for Trim pattern additions
- Maintains mutability for downstream Trim pattern code

**Test**:
- cargo build --release: ✅ PASS
- cargo test --release: ✅ 724/804 PASS (+4 improvements)
- Unit tests: ✅ 4/4 PASS

Phase 171-172 Stage 1: Total 66 lines reduced so far (Issue 4: 30 lines + Issue 5: 36 lines)

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

@@ -26,9 +26,11 @@
 //!
 //! Phase 171-172: Refactoring Infrastructure
 //! - loop_scope_shape_builder.rs: Unified LoopScopeShape initialization (Issue 4)
+//! - condition_env_builder.rs: Unified ConditionEnv construction (Issue 5)
 
 pub(in crate::mir::builder) mod ast_feature_extractor;
 pub(in crate::mir::builder) mod common_init;
+pub(in crate::mir::builder) mod condition_env_builder;
 pub(in crate::mir::builder) mod conversion_pipeline;
 pub(in crate::mir::builder) mod exit_binding;
 pub(in crate::mir::builder) mod loop_scope_shape_builder;