docs: Update control_flow module documentation (Phase 7)

- Enhanced module-level documentation in mod.rs
- Added comprehensive documentation to debug.rs
- Fixed unused imports in try_catch.rs
- Documented modularization history and architecture
- All visibility modifiers verified as correct
- Smoke tests pass (26/27, 1 unrelated timeout)

src/mir/builder/control_flow/debug.rs

@@ -1,4 +1,11 @@
-//! Debug utilities for control flow tracing
+//! Debug utilities for control flow tracing.
+//!
+//! This module provides diagnostic tools for tracing and debugging
+//! control flow behavior during MIR construction.
+//!
+//! # Environment Variables
+//!
+//! - `NYASH_TRACE_VARMAP=1` - Enable variable map tracing
 
 use super::super::MirBuilder;