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/exception/try_catch.rs

@@ -4,7 +4,7 @@
 //! including proper handling of deferred returns and cleanup blocks.
 
 use crate::ast::ASTNode;
-use crate::mir::builder::{Effect, EffectMask, MirInstruction, ValueId};
+use crate::mir::builder::{MirInstruction, ValueId};
 
 /// Control-flow: try/catch/finally
 ///