feat(joinir): Phase 135 P1 - Contract checks Fail-Fast (二度と破れない設計)

## Summary
Adds early Fail-Fast contract verification to prevent Phase 135 P0 issues from recurring.
Two new verifiers catch allocator SSOT violations and boundary inconsistencies before --verify.

## Changes

### Step 1: verify_condition_bindings_consistent
**Location**: `src/mir/builder/control_flow/joinir/merge/contract_checks.rs`

**Contract**: condition_bindings can have aliases (multiple names for same join_value),
but same join_value with different host_value is a violation.

**Example Error**:
```
[JoinIRVerifier/Phase135-P1] condition_bindings conflict:
  join_value ValueId(104) mapped to both ValueId(12) and ValueId(18)
```

**Catches**: ConditionLoweringBox bypassing SSOT allocator before BoundaryInjector

### Step 2: verify_header_phi_dsts_not_redefined
**Location**: `src/mir/builder/control_flow/joinir/merge/contract_checks.rs`

**Contract**: Loop header PHI dst ValueIds must not be reused as dst in non-PHI instructions.
Violation breaks MIR SSA (PHI dst overwrite).

**Example Error**:
```
[JoinIRVerifier/Phase135-P1] Header PHI dst ValueId(14) redefined by non-PHI instruction in block 3:
  Instruction: Call { dst: Some(ValueId(14)), ... }
```

**Catches**: ValueId collisions between header PHI dsts and lowered instructions

### Integration
**Location**: `src/mir/builder/control_flow/joinir/merge/mod.rs`

Added to `verify_joinir_contracts()`:
1. Step 1 runs before merge (validates boundary)
2. Step 2 runs after merge (validates func with PHI dst set)

### Documentation
- Updated `phase135_trim_mir_verify.sh` - Added P1 contract_checks description
- Updated `phase-135/README.md` - Added P1 section with contract details and effects

## Acceptance
✅ Build: SUCCESS
✅ Smoke: phase135_trim_mir_verify.sh - PASS
✅ Regression: phase132_exit_phi_parity.sh - 3/3 PASS
✅ Regression: phase133_json_skip_whitespace_llvm_exe.sh - PASS

## Effect
- **Prevention**: Future Box implementations catch SSOT violations immediately
- **Explicit Errors**: Phase 135-specific messages instead of generic --verify failures
- **Unbreakable**: Debug builds always detect violations, enforced by CI/CD

🤖 Generated with Claude Code

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

docs/development/current/main/phases/phase-135/README.md

@@ -1,8 +1,8 @@
 # Phase 135: ConditionLoweringBox allocator SSOT（ValueId 衝突の根治）
 
 ## Status
-- 状態: ✅ 実装完了（動作確認はローカルで実施）
-- スコープ: JoinIR の条件 lowering が `JoinValueSpace` と同一の allocator（SSOT）を使うことを保証する
+- 状態: ✅ P0 + P1 実装完了
+- スコープ: JoinIR の条件 lowering が `JoinValueSpace` と同一の allocator（SSOT）を使うことを保証 + contract_checks で Fail-Fast 強化
 
 ## Problem
 `apps/tests/phase133_json_skip_whitespace_min.hako` などで `--verify` が失敗し、MIR に以下の SSA 破綻が出ることがあった:
@@ -21,7 +21,60 @@
   - `condition_lowerer::lower_condition_to_joinir` は `&mut dyn FnMut() -> ValueId` を受理する。
 - `BoundaryInjector` は `condition_bindings` 注入を `dst` で重複排除し、異なる source が同一 dst に来る場合は Fail-Fast。
 
-## Acceptance
+## Acceptance (P0)
 - `./target/release/hakorune --verify apps/tests/phase133_json_skip_whitespace_min.hako` が PASS
 - `./target/release/hakorune --dump-mir apps/tests/phase133_json_skip_whitespace_min.hako` のループ header で PHI dst の再定義がない
 
+---
+
+## P1: Contract Checks 強化（Fail-Fast）
+
+### 目的
+Phase 135 P0 の根治を「二度と破れない」ように、JoinIR merge 時の contract violation を早期検出する Fail-Fast を追加。
+
+### 実装内容
+
+#### Step 1: `verify_condition_bindings_consistent`
+**場所**: `src/mir/builder/control_flow/joinir/merge/contract_checks.rs`
+
+**契約**:
+- condition_bindings は alias（同一 join_value に複数名）を許容
+- ただし、同一 join_value が異なる host_value に紐付く場合は即座に Fail-Fast
+
+**検出例**:
+```text
+[JoinIRVerifier/Phase135-P1] condition_bindings conflict:
+  join_value ValueId(104) mapped to both ValueId(12) and ValueId(18)
+  Contract: Same join_value can have multiple names (alias) but must have same host_value.
+```
+
+#### Step 2: `verify_header_phi_dsts_not_redefined`
+**場所**: `src/mir/builder/control_flow/joinir/merge/contract_checks.rs`
+
+**契約**:
+- Loop header PHI dst ValueIds は、PHI 以外の命令で dst として再利用してはいけない
+- 違反すると MIR SSA 破壊（PHI dst overwrite）
+
+**検出例**:
+```text
+[JoinIRVerifier/Phase135-P1] Header PHI dst ValueId(14) redefined by non-PHI instruction in block 3:
+  Instruction: Call { dst: Some(ValueId(14)), ... }
+  Contract: Header PHI dsts must not be reused as dst in other instructions.
+```
+
+### 統合
+`src/mir/builder/control_flow/joinir/merge/mod.rs` の `verify_joinir_contracts()` に統合：
+1. Step 1 を merge 前に実行（boundary 検証）
+2. Step 2 を merge 後に実行（func 検証、header PHI dst set を収集）
+
+### Acceptance (P1)
+- ✅ `cargo build --release` 成功
+- ✅ `phase135_trim_mir_verify.sh` - PASS
+- ✅ 回帰テスト: `phase132_exit_phi_parity.sh` - 3/3 PASS
+- ✅ 回帰テスト: `phase133_json_skip_whitespace_llvm_exe.sh` - PASS
+
+### 効果
+- **予防**: 今後の Box 実装で allocator SSOT 違反や boundary 矛盾を即座に検出
+- **明示的エラー**: `--verify` の汎用的なエラーではなく、Phase 135 固有の契約違反メッセージを出力
+- **二度と破れない**: debug build で必ず検出されるため、CI/CD で確実に防げる
+