phase29af(p0): pattern2 boundary hygiene ssot

src/mir/builder/control_flow/joinir/patterns/pattern2/README.md

@@ -27,6 +27,14 @@
 - `CarrierInit::BoolConst(_)` / `CarrierInit::LoopLocalZero` -> host binding is skipped
 - ConditionOnly carriers must not use `FromHost`
 
+## Boundary hygiene (Phase 29af)
+- Header PHI 対象: `carrier_info` の carriers（LoopState + ConditionOnly + LoopLocalZero）
+- Exit reconnection 対象: LoopState のみ（ConditionOnly は exit_bindings に入れない）
+- Host binding 対象: `CarrierInit::FromHost` のみ（BoolConst / LoopLocalZero は host slot 不要）
+- Fail-Fast: exit_bindings の `carrier_name` 重複は禁止（debug_assert）
+- Fail-Fast: `CarrierInit::FromHost` が `host_id=0` の場合は Err
+- SSOT: `docs/development/current/main/phases/phase-29af/README.md`
+
 ## Out of scope
 - multiple breaks / continue / return in the loop body
 - reassigned LoopBodyLocal outside the derived-slot shape

src/mir/builder/control_flow/joinir/patterns/pattern2_steps/emit_joinir_step_box.rs

@@ -66,7 +66,25 @@ impl EmitJoinIRStepBox {
 
         let exit_meta = &fragment_meta.exit_meta;
         use crate::mir::builder::control_flow::joinir::merge::exit_line::ExitMetaCollector;
-        let exit_bindings = ExitMetaCollector::collect(builder, exit_meta, Some(&inputs.carrier_info), debug);
+        let mut exit_bindings =
+            ExitMetaCollector::collect(builder, exit_meta, Some(&inputs.carrier_info), debug);
+        // Phase 29af P0: Exit reconnection targets LoopState only.
+        exit_bindings.retain(|binding| {
+            binding.role == crate::mir::join_ir::lowering::carrier_info::CarrierRole::LoopState
+        });
+        // Phase 29af P0: Reject duplicate carrier_name in exit_bindings.
+        #[cfg(debug_assertions)]
+        {
+            use std::collections::HashSet;
+            let mut seen = HashSet::new();
+            for binding in &exit_bindings {
+                debug_assert!(
+                    seen.insert(&binding.carrier_name),
+                    "Phase 29af Fail-Fast: duplicate exit_binding carrier '{}'",
+                    binding.carrier_name
+                );
+            }
+        }
 
         // Phase 256.8.5: Use JoinModule.entry.params as SSOT (no hardcoded ValueIds)
         use super::super::common::get_entry_function;
@@ -78,7 +96,22 @@ impl EmitJoinIRStepBox {
         // Build host_input_values in same order (loop_var + carriers)
         let mut host_input_values = vec![inputs.loop_var_id];
         for carrier in inputs.carrier_info.carriers.iter() {
-            host_input_values.push(carrier.host_id);
+            use super::super::common::{decide_carrier_binding_policy, CarrierBindingPolicy};
+            match decide_carrier_binding_policy(carrier) {
+                CarrierBindingPolicy::BindFromHost => {
+                    if carrier.host_id == crate::mir::ValueId(0) {
+                        return Err(format!(
+                            "[emit_joinir] Phase 29af Fail-Fast: FromHost carrier '{}' has host_id=0",
+                            carrier.name
+                        ));
+                    }
+                    host_input_values.push(carrier.host_id);
+                }
+                CarrierBindingPolicy::SkipBinding => {
+                    // Placeholder: SkipBinding does not require a host slot.
+                    host_input_values.push(crate::mir::ValueId(0));
+                }
+            }
         }
 
         // Verify count consistency (fail-fast)