refactor(joinir): Document condition lowerer architecture (Task 1)

Task 1: Condition lowerer integration investigation
- Analyzed 3 modules: condition_lowerer (522L), bool_expr_lowerer (371L), condition_env_builder (248L)
- Key finding: Cannot merge safely due to different design patterns:
  - condition_lowerer: JoinIR-based (pure functional, no builder state)
  - bool_expr_lowerer: MIR-based (stateful MirBuilder)
- Added clear TODO comment documenting the distinction
- bool_expr_lowerer appears unused (all tests commented), candidate for future removal

Decision: Keep separate, document architecture clearly for future maintainers

src/mir/join_ir/lowering/bool_expr_lowerer.rs

@@ -33,6 +33,15 @@
 //! %or2 = BinOp Or %or1 %cmp3
 //! %result = BinOp Or %or2 %cmp4
 //! ```
+//!
+//! ## Status (Phase 179)
+//!
+//! **TODO: Consider removal or unification** - This module appears unused (all tests commented).
+//! Cannot easily merge with `condition_lowerer.rs` because:
+//! - This module: MIR-based (uses MirBuilder, stateful)
+//! - condition_lowerer: JoinIR-based (pure functional, no builder)
+//!
+//! Decision: Keep separate for now, document the distinction clearly.
 
 use crate::ast::{ASTNode, BinaryOperator, UnaryOperator};
 use crate::mir::builder::MirBuilder;