feat(joinir): Phase 172-3 ExitMeta unified return from Pattern2 lowerer

Implement loop exit contract boxification for JoinIR Pattern2:

- lower_loop_with_break_minimal now returns (JoinModule, ExitMeta)
- ExitMeta contains k_exit parameter ValueId for carrier variables
- Pattern2 caller builds exit_bindings from ExitMeta
- merge/mod.rs adds exit_bindings join_exit_values to used_values for remap
- reconnect_boundary uses remapped exit values for variable_map updates

This completes Phases 172-3 through 172-5 of the Loop Exit Contract
boxification plan, enabling proper loop variable propagation after exit.

Test: joinir_min_loop.hako passes (RC: 0)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/mir/builder/control_flow/joinir/merge/mod.rs

@@ -88,6 +88,17 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
             }
             used_values.insert(binding.join_value);
         }
+
+        // Phase 172-3: Add exit_bindings' join_exit_values to used_values for remapping
+        for binding in &boundary.exit_bindings {
+            if debug {
+                eprintln!(
+                    "[cf_loop/joinir] Phase 172-3: Adding exit binding '{}' JoinIR {:?} to used_values",
+                    binding.carrier_name, binding.join_exit_value
+                );
+            }
+            used_values.insert(binding.join_exit_value);
+        }
     }
 
     // Phase 3: Remap ValueIds

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

@@ -141,9 +141,10 @@ impl MirBuilder {
             variable_definitions: BTreeMap::new(),
         };
 
-        // Phase 169 / Phase 171-fix: Call Pattern 2 lowerer with ConditionEnv
-        let join_module = match lower_loop_with_break_minimal(scope, condition, &env) {
-            Ok(module) => module,
+        // Phase 169 / Phase 171-fix / Phase 172-3: Call Pattern 2 lowerer with ConditionEnv
+        // Now returns (JoinModule, ExitMeta) for proper exit_bindings construction
+        let (join_module, exit_meta) = match lower_loop_with_break_minimal(scope, condition, &env, &loop_var_name) {
+            Ok((module, meta)) => (module, meta),
             Err(e) => {
                 // Phase 195: Use unified trace
                 trace::trace().debug("pattern2", &format!("Pattern 2 lowerer failed: {}", e));
@@ -174,20 +175,43 @@ impl MirBuilder {
         );
 
         // Merge JoinIR blocks into current function
-        // Phase 171-fix: Create boundary with ConditionBindings
-        let boundary = if condition_bindings.is_empty() {
-            crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary::new_inputs_only(
+        // Phase 172-3/4: Create boundary with exit_bindings from ExitMeta
+        //
+        // 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 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 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(
             vec![ValueId(0)],      // JoinIR's main() parameter (loop variable init)
             vec![loop_var_id],     // Host's loop variable
-            )
-        } else {
-            crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary::new_with_condition_bindings(
-                vec![ValueId(0)],      // JoinIR's main() parameter (loop variable init)
-                vec![loop_var_id],     // Host's loop variable
-                condition_bindings,     // Phase 171-fix: ConditionBindings with JoinIR ValueIds
-            )
-        };
-        // Phase 189: Discard exit PHI result (Pattern 2 returns void)
+        );
+        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)?;
 
         // Phase 188-Impl-2: Return Void (loops don't produce values)

src/mir/join_ir/lowering/loop_with_break_minimal.rs

@@ -56,6 +56,7 @@
 //! Following the "80/20 rule" from CLAUDE.md - get it working first, generalize later.
 
 use crate::ast::ASTNode;
+use crate::mir::join_ir::lowering::carrier_info::ExitMeta;
 use crate::mir::join_ir::lowering::condition_to_joinir::{lower_condition_to_joinir, ConditionEnv};
 use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
 use crate::mir::join_ir::{
@@ -106,11 +107,21 @@ use crate::mir::ValueId;
 ///
 /// The caller must build a ConditionEnv that maps variable names to JoinIR-local ValueIds.
 /// This ensures JoinIR never accesses HOST ValueIds directly.
+///
+/// # Phase 172-3: ExitMeta Return
+///
+/// Returns `(JoinModule, ExitMeta)` where ExitMeta contains the JoinIR-local ValueId
+/// of the k_exit parameter. This allows the caller to build proper exit_bindings.
+///
+/// # Arguments
+///
+/// * `loop_var_name` - Name of the loop variable (for ExitMeta construction)
 pub fn lower_loop_with_break_minimal(
     _scope: LoopScopeShape,
     condition: &ASTNode,
     env: &ConditionEnv,
-) -> Result<JoinModule, String> {
+    loop_var_name: &str,
+) -> Result<(JoinModule, ExitMeta), String> {
     // Phase 188-Impl-2: Use local ValueId allocator (sequential from 0)
     // JoinIR has NO knowledge of host ValueIds - boundary handled separately
     let mut value_counter = 0u32;
@@ -294,5 +305,9 @@ pub fn lower_loop_with_break_minimal(
     eprintln!("[joinir/pattern2] Condition from AST (not hardcoded)");
     eprintln!("[joinir/pattern2] Exit PHI: k_exit receives i from both natural exit and break");
 
-    Ok(join_module)
+    // Phase 172-3: Build ExitMeta with k_exit parameter's ValueId
+    let exit_meta = ExitMeta::single(loop_var_name.to_string(), i_exit);
+    eprintln!("[joinir/pattern2] Phase 172-3: ExitMeta {{ {} → {:?} }}", loop_var_name, i_exit);
+
+    Ok((join_module, exit_meta))
 }