feat(joinir): Phase 228 - CarrierInit for ConditionOnly header PHI initialization

- Add CarrierInit enum (FromHost/BoolConst) for explicit initialization policy
- LoopHeaderPhiBuilder generates Const instruction for BoolConst carriers
- merge/mod.rs uses carrier_info to include ALL carriers in header PHI
- Pattern 2/4 pass carrier_info to boundary builder
- ConditionOnly carriers (is_digit_pos) now included in header PHI

Remaining: latch incoming for ConditionOnly carriers (Phase 228-8)

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -141,12 +141,23 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
                 "Phase 201-A: No host_inputs in boundary for loop_var_init"
             )?;
 
-            // Extract other carriers from exit_bindings
-            let other_carriers: Vec<(String, ValueId)> = boundary.exit_bindings
-                .iter()
-                .filter(|b| b.carrier_name != *loop_var_name)
-                .map(|b| (b.carrier_name.clone(), b.host_slot))
-                .collect();
+            // Phase 228-4: Extract carriers with their initialization strategy
+            let other_carriers: Vec<(String, ValueId, crate::mir::join_ir::lowering::carrier_info::CarrierInit, crate::mir::join_ir::lowering::carrier_info::CarrierRole)> =
+                if let Some(ref carrier_info) = boundary.carrier_info {
+                    // Use carrier_info if available (Phase 228)
+                    carrier_info.carriers
+                        .iter()
+                        .filter(|c| c.name != *loop_var_name)
+                        .map(|c| (c.name.clone(), c.host_id, c.init, c.role))
+                        .collect()
+                } else {
+                    // Fallback: exit_bindings から取得（既存動作）
+                    boundary.exit_bindings
+                        .iter()
+                        .filter(|b| b.carrier_name != *loop_var_name)
+                        .map(|b| (b.carrier_name.clone(), b.host_slot, crate::mir::join_ir::lowering::carrier_info::CarrierInit::FromHost, crate::mir::join_ir::lowering::carrier_info::CarrierRole::LoopState))
+                        .collect()
+                };
 
             if debug {
                 eprintln!(
@@ -156,7 +167,7 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
                 eprintln!(
                     "[cf_loop/joinir]   loop_var_init={:?}, carriers={:?}",
                     loop_var_init,
-                    other_carriers.iter().map(|(n, _)| n.as_str()).collect::<Vec<_>>()
+                    other_carriers.iter().map(|(n, _, _, _)| n.as_str()).collect::<Vec<_>>()
                 );
             }