refactor(llvm): Phase 131-12-P2 - block_end_values SSOT 化（WIP）

## 実装内容
- get_end_values() API 追加
- _value_at_end_i64() を snapshot-only に変更
- def_blocks 即時更新
- PHI incoming を snapshot から取得

## 発見された問題
- 同一ブロック内の def→use が predecessor snapshot を見てしまう
- これは次フェーズで resolve_cur / resolve_incoming 分離で修正

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

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

src/llvm_py/builders/block_lower.py

@@ -245,6 +245,12 @@ def lower_blocks(builder, func: ir.Function, block_by_id: Dict[int, Dict[str, An
                             vmap_cur[dst] = _gval
                     if dst not in created_ids and dst in vmap_cur:
                         created_ids.append(dst)
+                        # P0-1.5: Update def_blocks IMMEDIATELY after instruction lowering
+                        # This ensures resolver can detect defined_here for same-block uses
+                        try:
+                            builder.def_blocks.setdefault(dst, set()).add(block_data.get("id", 0))
+                        except Exception:
+                            pass
             except Exception:
                 pass
         # Phase 131-4 Pass A: DEFER terminators until after PHI finalization