# Phase 29af P0: Pattern2 Boundary Hygiene (SSOT)

Goal: Pattern2 の boundary 情報の歪みを SSOT で整理し、将来の回帰を防ぐ（仕様不変）。

## Boundary Contract (SSOT)

- 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 Rules

- exit_bindings の `carrier_name` 重複は禁止（debug_assert）
- `CarrierInit::FromHost` の `host_id=0` は Fail-Fast

## Entry Points

- boundary 構築: `src/mir/builder/control_flow/joinir/patterns/pattern2_steps/emit_joinir_step_box.rs`
- header PHI 事前構築: `src/mir/builder/control_flow/joinir/merge/header_phi_prebuild.rs`
- exit_bindings 収集: `src/mir/builder/control_flow/joinir/merge/exit_line/meta_collector.rs`
- latch 記録: `src/mir/builder/control_flow/joinir/merge/rewriter/{tail_call_policy,latch_incoming_recorder}.rs`

## Verification

- `cargo build --release`
- `./tools/smokes/v2/run.sh --profile quick`
- `./tools/smokes/v2/run.sh --profile integration --filter "phase29ab_pattern2_"`
- `./tools/smokes/v2/run.sh --profile integration --filter "phase1883_"`

## Notes

- Merge 側の Header PHI Entry/Latch contract は Phase 29ae で SSOT 化済み: `docs/development/current/main/phases/phase-29ae/README.md`