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).

🐱✨

src/runtime/mod.rs

@@ -6,6 +6,7 @@ pub mod box_registry;
 pub mod core_box_ids; // Phase 87: CoreBoxId/CoreMethodId 型安全enum
 pub mod deprecations;
 pub mod gc;
+pub mod ring0; // Phase 88: Ring0Context - OS API 抽象化レイヤー
 pub mod gc_controller;
 pub mod gc_mode;
 pub mod gc_trace;
@@ -38,6 +39,7 @@ mod tests;
 pub use box_registry::{get_global_registry, BoxFactoryRegistry, BoxProvider};
 pub use core_box_ids::{CoreBoxCategory, CoreBoxId, CoreMethodId}; // Phase 87: 型安全enum
 pub use plugin_config::PluginConfig;
+pub use ring0::{get_global_ring0, init_global_ring0, Ring0Context}; // Phase 88: Ring0 公開 API
 pub use plugin_loader_unified::{
     get_global_plugin_host, init_global_plugin_host, MethodHandle, PluginBoxType, PluginHost,
     PluginLibraryHandle,