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

src/runtime/ring0/mod.rs

@@ -7,7 +7,7 @@ mod std_impls;
 mod traits;
 
 pub use errors::{IoError, TimeError};
-pub use std_impls::{NoopMem, StdFs, StdIo, StdLog, StdThread, StdTime};
+pub use std_impls::{NoopMem, StdFs, StdIo, StdLog, StdMem, StdThread, StdTime};
 pub use traits::{
     FsApi, FsMetadata, IoApi, LogApi, LogLevel, MemApi, MemStats, ThreadApi, TimeApi,
 };
@@ -63,7 +63,7 @@ impl std::fmt::Debug for Ring0Context {
 /// デフォルト Ring0Context を作成（std ベース）
 pub fn default_ring0() -> Ring0Context {
     Ring0Context {
-        mem: Arc::new(NoopMem),
+        mem: Arc::new(StdMem::new()),
         io: Arc::new(StdIo),
         time: Arc::new(StdTime),
         log: Arc::new(StdLog),
@@ -128,4 +128,20 @@ mod tests {
         ring0.log.warn("warn message");
         ring0.log.error("error message");
     }
+
+    #[test]
+    fn test_default_ring0_uses_stdmem() {
+        let ring0 = default_ring0();
+        let ptr = ring0.mem.alloc(512);
+        assert!(!ptr.is_null(), "default_ring0 should use StdMem");
+        ring0.mem.free(ptr);
+
+        // Clean up
+        unsafe {
+            std::alloc::dealloc(
+                ptr,
+                std::alloc::Layout::from_size_align_unchecked(512, 1),
+            )
+        };
+    }
 }