feat(ring0): Phase 102 MemApi Bridge Skeleton - StdMem実装

- Add StdMem struct (stdlib alloc/free based)
- Update default_ring0() to use StdMem instead of NoopMem
- Keep NoopMem for compatibility and testing
- Add unit tests for StdMem allocation/stats
- Update docs (phase-85-ring0-runtime/README.md, ring0-inventory.md)

Note: This is a skeleton implementation. Full MemStats tracking
(including freed size) will be added in Phase 102B with hakmem bridge.

Files modified:
- src/runtime/ring0/std_impls.rs
- src/runtime/ring0/mod.rs
- docs/development/current/main/phase-85-ring0-runtime/README.md
- docs/development/current/main/ring0-inventory.md

docs/development/current/main/phase-85-ring0-runtime/README.md

@@ -0,0 +1,75 @@
+# Phase 85: Ring0 Runtime System
+
+Ring0 is the OS API abstraction layer in Hakorune/Nyash, providing a clean interface to system resources without knowing about Boxes or Nyash-specific concepts.
+
+## Architecture
+
+Ring0Context bundles multiple API traits:
+- **MemApi**: Memory allocation (malloc/free)
+- **IoApi**: Stdin/Stdout/Stderr operations
+- **TimeApi**: System time and monotonic clock
+- **LogApi**: Structured logging
+- **FsApi**: File system operations (Phase 90-A)
+- **ThreadApi**: Thread operations (Phase 90-D)
+
+## Implementations
+
+### Phase 88: Initial std-based implementations
+- NoopMem: Stub memory API (returns null)
+- StdIo: std::io based I/O
+- StdTime: std::time based time
+- StdLog: eprintln!/println! based logging
+- StdFs: std::fs based filesystem
+- StdThread: std::thread based threading
+
+### Phase 102: MemApi Bridge Skeleton
+
+**Completed Tasks**:
+- StdMem implementation (stdlib alloc/free based)
+- default_ring0() unified to use StdMem instead of NoopMem
+- Unit tests for StdMem allocation and statistics
+- NoopMem retained for compatibility
+
+#### Implementation Status
+- **StdMem**: Implemented (stdlib alloc/free based)
+- **Statistics Management**: allocated/freed/current counters
+- **Compatibility**: NoopMem retained for testing
+
+#### Design Notes
+- The free() method counts freed operations but not freed bytes (no size info)
+- Full statistics tracking will be added in Phase 102B (hakmem integration)
+
+#### Usage Example
+```rust
+let mem = StdMem::new();
+let ptr = mem.alloc(1024);
+let stats = mem.stats();
+println!("allocated: {}, current: {}", stats.allocated, stats.current);
+mem.free(ptr);
+```
+
+#### Next Steps
+Phase 102B will add:
+- Size-pairing dictionary (record alloc sizes)
+- Accurate freed size statistics
+- hakmem API bridge integration
+
+## Global Ring0Context
+
+Ring0 provides a global singleton via:
+- `init_global_ring0(ctx)`: Initialize global context
+- `get_global_ring0()`: Get global context
+
+## Testing
+
+All Ring0 components have comprehensive unit tests. Run with:
+```bash
+cargo test --release --lib ring0
+```
+
+## Files
+
+- `src/runtime/ring0/mod.rs`: Main module and Ring0Context struct
+- `src/runtime/ring0/traits.rs`: API trait definitions
+- `src/runtime/ring0/std_impls.rs`: Standard library implementations
+- `src/runtime/ring0/errors.rs`: Error types