refactor: unify string helpers and pattern2 derived slot

src/mir/builder/control_flow/normalization/README.md

@@ -122,7 +122,9 @@ pub enum PlanKind {
 
 ### Phase 142 P0: Statement-Level Normalization **CURRENT**
 - **Change**: Normalization unit changed from "block suffix" to "statement (loop only)"
-- **Pattern**: `loop(true) { ... break }` - always returns `LoopOnly`, regardless of subsequent statements
+- **Pattern**: `loop(true)` with **Normalized-supported body shapes** only
+  - Body ends with `break` and prior statements are `assignment`/`local` only
+  - Body is a single `if` with `break`/`continue` branches (optional else)
 - **Consumed**: Always 1 statement (the loop itself)
 - **Kind**: `PlanKind::LoopOnly`
 - **Subsequent statements**: Handled by normal MIR lowering (not normalized)