feat(phase88): Ring0Context Skeleton implementation

Phase 88: OS API abstraction layer implementation

**Implementation**:
- Ring0Context module added (4 files)
  - mod.rs: Public API, global initialization (OnceLock)
  - traits.rs: MemApi, IoApi, TimeApi, LogApi trait definitions
  - std_impls.rs: std-based default implementations
  - errors.rs: IoError, TimeError type definitions

**Design Principles**:
- Ring0 knows nothing about Box
- Ring0 knows nothing about Nyash
- Pure OS API abstraction

**Global Initialization**:
- NyashRunner::new() initializes Ring0Context globally
- OnceLock ensures safe initialization (idempotent)

**Migration (2 paths)**:
- src/runner/selfhost.rs:27: eprintln! → ring0.log.error() (OOB strict)
- src/runner/selfhost.rs:177: eprintln! → ring0.log.error() (PyVM error)

**Tests**:
- 4 unit tests added (ring0 module)
- All tests passed
- Build successful (0 errors)

**Migration Status**:
- Migrated: 2/3,955 (0.05%)
- Remaining: 3,953 paths (Phase 89+)

**Files Changed**:
- src/runtime/ring0/mod.rs (new, 100 lines)
- src/runtime/ring0/traits.rs (new, 93 lines)
- src/runtime/ring0/std_impls.rs (new, 77 lines)
- src/runtime/ring0/errors.rs (new, 26 lines)
- src/runtime/mod.rs (Ring0Context export)
- src/runner/mod.rs (global initialization)
- src/runner/selfhost.rs (2 paths migrated)
- docs/development/current/main/ring0-inventory.md (Phase 88 status)

Phase 88 complete. Ready for Phase 89 (gradual migration).

🐱✨

docs/development/current/main/ring0-inventory.md

@@ -14,6 +14,24 @@
   - 将来的には `Ring0Context.log` / `Ring0Context.io` 経由に寄せる。
   - 代表パス（selfhost/hack_check/VM/LLVM）の `println!/eprintln!` から段階的に移行する。
 
+### Phase 88 移行状況（2025-12-02）
+
+**移行済みパス（2箇所）**:
+
+| ファイル | 行 | Before | After |
+|---------|---|--------|-------|
+| `src/runner/selfhost.rs` | 27 | `eprintln!("[selfhost][oob-strict] ...")` | `ring0.log.error("[selfhost][oob-strict] ...")` |
+| `src/runner/selfhost.rs` | 177 | `eprintln!("❌ PyVM error ...")` | `ring0.log.error("❌ PyVM error ...")` |
+
+**残存パス（3,953箇所）**:
+
+| カテゴリ | 出現回数 | 移行予定 |
+|---------|---------|---------|
+| println! | 2,200 | Phase 89-A |
+| eprintln! | 1,753 | Phase 89-B |
+
+Phase 89 以降で段階的に移行予定。
+
 ---
 
 ## 2. Box / プラグイン / カーネル実装の数