refactor(joinir): Unify exit binding generation using ExitMetaCollector

Phase 179 Task 2: Modular exit binding generation

Changes:
- Pattern 4 (pattern4_with_continue.rs):
  - REPLACED: Manual exit_bindings loop (28 lines deleted)
  - WITH: ExitMetaCollector::collect() call (10 lines added)
  - ADDED: Validation that all carriers have bindings
  - Net reduction: 10 lines, improved modularity

- Pattern 3 (pattern3_with_if_phi.rs):
  - DOCUMENTED: Why hardcoded ValueId(18) exists
  - ADDED: TODO with migration path to ExitMeta-based approach
  - DEFERRED: Actual migration (requires lowerer return type change)
  - Impact: Low priority - Pattern 3 is test-only

Benefits:
- Unified approach: Pattern 4 now uses same ExitMetaCollector as other patterns
- Reusability: ExitMetaCollector is pattern-agnostic Box
- Maintainability: Less duplicate code, clearer responsibilities
- Testability: ExitMetaCollector has independent unit tests

Notes:
- Pattern 3 migration deferred (lowerer doesn't return ExitMeta yet)
- Comprehensive TODO comment documents migration path
- Pre-existing test failure (stage1_usingresolver) unrelated to this change

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

@@ -7,6 +7,23 @@ use super::super::trace;
 
 /// Phase 179-A: Expected ValueId for k_exit parameter (sum_final) in Pattern 3
 /// This corresponds to the exit PHI input in the JoinIR lowering for loop_with_if_phi_minimal
+///
+/// # TODO (Phase 179 Task 2): Convert to ExitMeta-based exit binding generation
+///
+/// **Current State**: Hardcoded ValueId(18) - fragile and non-reusable
+///
+/// **Why it's hardcoded**:
+/// - Pattern 3's lowerer (`lower_loop_with_if_phi_pattern`) returns `Option<JoinModule>`
+/// - Unlike Pattern 4 which returns `(JoinModule, JoinFragmentMeta)`
+/// - No ExitMeta available to dynamically look up exit PHI ValueIds
+///
+/// **Migration Path** (when Pattern 3 lowerer is updated):
+/// 1. Change `lower_loop_with_if_phi_pattern` to return `(JoinModule, JoinFragmentMeta)`
+/// 2. Remove this constant
+/// 3. Use ExitMeta loop (like Pattern 4 lines 350-378) to generate exit_bindings dynamically
+/// 4. See: pattern4_with_continue.rs lines 350-378 for reference implementation
+///
+/// **Impact**: Low priority - Pattern 3 is test-only and works correctly with hardcoded value
 const PATTERN3_K_EXIT_SUM_FINAL_ID: ValueId = ValueId(18);
 
 /// Phase 194: Detection function for Pattern 3

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

@@ -347,34 +347,24 @@ impl MirBuilder {
             );
         }
 
-        // Phase 196: Dynamically generate exit_bindings from ExitMeta and CarrierInfo
-        let mut exit_bindings = Vec::new();
-        for (carrier_name, join_exit_value) in &exit_meta.exit_values {
-            // Find the host_slot for this carrier
-            let host_slot = carrier_info
-                .carriers
-                .iter()
-                .find(|c| &c.name == carrier_name)
-                .map(|c| c.host_id)
-                .ok_or_else(|| {
-                    format!(
-                        "[cf_loop/pattern4] Carrier '{}' not found in CarrierInfo",
-                        carrier_name
-                    )
-                })?;
-
-            exit_bindings.push(
-                crate::mir::join_ir::lowering::inline_boundary::LoopExitBinding {
-                    carrier_name: carrier_name.clone(),
-                    join_exit_value: *join_exit_value,
-                    host_slot,
-                },
+        // Phase 179 Task 2: Use ExitMetaCollector for unified exit binding generation
+        // Replaces manual loop (Phase 196 implementation) with modular Box approach
+        use super::super::merge::exit_line::meta_collector::ExitMetaCollector;
+        let exit_bindings = ExitMetaCollector::collect(
+            self,
+            &exit_meta,
+            debug,
         );
 
-            trace::trace().debug(
-                "pattern4",
-                &format!("Generated binding: {} → JoinIR={} Host={}", carrier_name, join_exit_value.0, host_slot.0)
-            );
+        // Validate that all expected carriers have bindings
+        // (ExitMetaCollector silently skips missing carriers, but Pattern 4 expects all)
+        for carrier in &carrier_info.carriers {
+            if !exit_bindings.iter().any(|b| b.carrier_name == carrier.name) {
+                return Err(format!(
+                    "[cf_loop/pattern4] Carrier '{}' not found in variable_map - exit binding missing",
+                    carrier.name
+                ));
+            }
         }
 
         // Phase 196: Build host_inputs dynamically