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>

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

@@ -1065,9 +1065,20 @@ fn verify_joinir_contracts(
     loop_info: &LoopHeaderPhiInfo,
     boundary: &JoinInlineBoundary,
 ) {
+    // Phase 135 P1 Step 1: Verify condition_bindings consistency (before merge)
+    contract_checks::verify_condition_bindings_consistent(boundary);
+
     contract_checks::verify_loop_header_phis(func, header_block, loop_info, boundary);
     verify_no_phi_dst_overwrite(func, header_block, loop_info); // Phase 204-2
     verify_phi_inputs_defined(func, header_block); // Phase 204-3
     contract_checks::verify_exit_line(func, exit_block, boundary);
     contract_checks::verify_valueid_regions(loop_info, boundary); // Phase 205-4
+
+    // Phase 135 P1 Step 2: Verify header PHI dsts not redefined (after merge)
+    let phi_dsts: std::collections::HashSet<_> = loop_info
+        .carrier_phis
+        .values()
+        .map(|entry| entry.phi_dst)
+        .collect();
+    contract_checks::verify_header_phi_dsts_not_redefined(func, header_block, &phi_dsts);
 }