chore: Remove unused imports (Task 4)

Task 4: Additional dead code and unused imports cleanup
- Removed 7 unused imports across 6 files:
  - conversion_pipeline.rs: MirModule
  - pattern1_minimal.rs: BTreeMap
  - pattern2_with_break.rs: BTreeMap
  - pattern3_with_if_phi.rs: BTreeMap
  - pattern4_with_continue.rs: CarrierInfo, BTreeMap
  - condition_var_analyzer.rs: BasicBlockId
- Build verified: 0 unused import warnings remaining
- Warning count reduced: 70 → 63 warnings

Result: Cleaner code, reduced compiler warnings

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

@@ -27,7 +27,7 @@
 //! - **Testability**: Can test conversion independently
 //! - **Reduces duplication**: Eliminates 120 lines across Pattern 1-4
 
-use crate::mir::{MirModule, ValueId};
+use crate::mir::ValueId;
 use crate::mir::builder::MirBuilder;
 use crate::mir::join_ir::JoinModule;
 use crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary;

src/mir/builder/control_flow/joinir/patterns/pattern1_minimal.rs

@@ -52,7 +52,7 @@ impl MirBuilder {
     ) -> Result<Option<ValueId>, String> {
         use crate::mir::join_ir::lowering::simple_while_minimal::lower_simple_while_minimal;
         use crate::mir::BasicBlockId;
-        use std::collections::{BTreeMap, BTreeSet};
+        use std::collections::BTreeSet;
 
         // Phase 195: Use unified trace
         trace::trace().debug("pattern1", "Calling Pattern 1 minimal lowerer");

src/mir/builder/control_flow/joinir/patterns/pattern2_with_break.rs

@@ -95,7 +95,7 @@ impl MirBuilder {
     ) -> Result<Option<ValueId>, String> {
         use crate::mir::join_ir::lowering::loop_with_break_minimal::lower_loop_with_break_minimal;
         use crate::mir::BasicBlockId;
-        use std::collections::{BTreeMap, BTreeSet};
+        use std::collections::BTreeSet;
 
         // Phase 195: Use unified trace
         trace::trace().debug("pattern2", "Calling Pattern 2 minimal lowerer");

src/mir/builder/control_flow/joinir/patterns/pattern3_with_if_phi.rs

@@ -70,7 +70,7 @@ impl MirBuilder {
     ) -> Result<Option<ValueId>, String> {
         use crate::mir::join_ir::lowering::loop_with_if_phi_minimal::lower_loop_with_if_phi_pattern;
         use crate::mir::BasicBlockId;
-        use std::collections::{BTreeMap, BTreeSet};
+        use std::collections::BTreeSet;
 
         // Phase 195: Use unified trace
         trace::trace().debug("pattern3", "Calling Pattern 3 minimal lowerer");

src/mir/builder/control_flow/joinir/patterns/pattern4_with_continue.rs

@@ -168,10 +168,9 @@ impl MirBuilder {
         _func_name: &str,
         debug: bool,
     ) -> Result<Option<ValueId>, String> {
-        use crate::mir::join_ir::lowering::carrier_info::CarrierInfo;
         use crate::mir::join_ir::lowering::loop_with_continue_minimal::lower_loop_with_continue_minimal;
         use crate::mir::BasicBlockId;
-        use std::collections::{BTreeMap, BTreeSet};
+        use std::collections::BTreeSet;
         use super::pattern4_carrier_analyzer::Pattern4CarrierAnalyzer;
 
         // Phase 195: Use unified trace

src/mir/loop_pattern_detection/condition_var_analyzer.rs

@@ -15,7 +15,6 @@
 
 use crate::ast::ASTNode;
 use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
-use crate::mir::BasicBlockId;
 use std::collections::HashSet;
 
 /// Extract all variable names from an AST expression