feat(joinir/refactor): Phase 33-10-P1 ExitMetaCollector Box modularization

Box theory refactoring: Extract exit_bindings construction logic from
pattern lowerers into focused, reusable ExitMetaCollector Box.

**Changes**:
1. Created meta_collector.rs (+102 lines):
   - ExitMetaCollector::collect() builds exit_bindings from ExitMeta
   - Pure function (no side effects)
   - Reusable by all pattern lowerers

2. Updated pattern2_with_break.rs (-20 lines):
   - Use ExitMetaCollector::collect() instead of inline filter_map
   - Removed manual binding construction loop
   - Cleaner caller code

3. Made exit_line module public:
   - Allows pattern lowerers to use ExitMetaCollector
   - Clear module visibility boundaries

**Box Design**:
- Single responsibility: Convert ExitMeta + variable_map → exit_bindings
- Pure function: No side effects, testable independently
- Reusable: Can be used by Pattern 3, Pattern 4, etc.

**Testing**:
- Build: ✅ Success (1m 04s)
- Execution: ✅ RC: 0 (Pattern 2 verified)
- Regression: ✅ No issues

**Metrics**:
- New lines: +102 (meta_collector.rs)
- Removed lines: -20 (pattern2_with_break.rs)
- Net change: +82 lines
- Code clarity: Significantly improved

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

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

@@ -179,21 +179,11 @@ impl MirBuilder {
         //
         // The loop variable's exit value needs to be reflected back to variable_map.
         // ExitMeta provides the correct JoinIR-local ValueId for k_exit parameter.
-        use crate::mir::join_ir::lowering::inline_boundary::LoopExitBinding;
+        // Phase 33-10-Refactor-P1: Use ExitMetaCollector Box to build exit_bindings
+        use crate::mir::builder::control_flow::joinir::merge::exit_line::ExitMetaCollector;
 
-        // Phase 172-3: Build exit bindings from ExitMeta (provides actual k_exit parameter ValueId)
-        let exit_bindings: Vec<LoopExitBinding> = exit_meta.exit_values
-            .iter()
-            .filter_map(|(carrier_name, join_exit_value)| {
-                // Look up host slot from variable_map
-                let host_slot = self.variable_map.get(carrier_name).copied()?;
-                Some(LoopExitBinding {
-                    carrier_name: carrier_name.clone(),
-                    join_exit_value: *join_exit_value,
-                    host_slot,
-                })
-            })
-            .collect();
+        // Phase 33-10: Collect exit bindings from ExitMeta using Box
+        let exit_bindings = ExitMetaCollector::collect(self, &exit_meta, debug);
 
         // Phase 172-3: Build boundary with both condition_bindings and exit_bindings
         let mut boundary = crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary::new_inputs_only(
@@ -203,14 +193,6 @@ impl MirBuilder {
         boundary.condition_bindings = condition_bindings;
         boundary.exit_bindings = exit_bindings.clone();
 
-        // Phase 172-3 Debug: Log exit bindings from ExitMeta
-        for binding in &exit_bindings {
-            eprintln!(
-                "[cf_loop/pattern2] Phase 172-3: exit_binding '{}' JoinIR {:?} → HOST {:?}",
-                binding.carrier_name, binding.join_exit_value, binding.host_slot
-            );
-        }
-
         // Phase 189: Capture exit PHI result (now used for reconnect)
         let _ = self.merge_joinir_mir_blocks(&mir_module, Some(&boundary), debug)?;