feat(virtualfs): implement native filesystem backend

Adds a NativeFileSystem implementation that provides direct access to the
underlying filesystem, rooted at a specific directory. This implementation:

- Implements the VirtualFileSystem trait for native filesystem operations
- Handles file/directory creation, reading, writing, and metadata
- Supports symlinks and permissions on Unix systems
- Includes comprehensive test coverage
- Exposes filesystem constants as public in metadata.rs

The native filesystem backend allows direct filesystem access while maintaining
the VFS abstraction layer, enabling consistent filesystem operations across
different backend implementations.

README.md

@@ -95,6 +95,7 @@
 |                   | **OverlayFS**          |   ✅   | OverlayFS implementation on macOS                        |
 |                   | • Core                 |   ✅   | Core implementation of the OverlayFS                     |
 |                   | • NFS                  |   ✅   | Network File System server implementation                |
+|                   | • NativeFS             |   ✅   | Native filesystem implementation                         |
 |                   | • virtiofs             |  ⬜️   | libkrun virtiofs implementation                          |
 |                   | Sandboxes Registry     |  ⬜️   | Container sandboxing registry implementation             |
 |                   | Docker Registry        |   ✅   | Integration with Docker registry                         |

monocore/lib/oci/implementations/docker.rs

@@ -336,9 +336,7 @@ impl OciRegistryPull for DockerRegistry {
             .zip(config.rootfs().diff_ids())
             .map(|(layer_desc, diff_id)| async {
                 // Check if layer already exists in database
-                if management::layer_exists(&self.oci_db, &layer_desc.digest().to_string())
-                    .await?
-                {
+                if management::layer_exists(&self.oci_db, &layer_desc.digest().to_string()).await? {
                     tracing::info!(
                         "layer {} already exists, skipping download",
                         layer_desc.digest()

virtualfs/Cargo.toml

@@ -30,3 +30,4 @@ tracing-subscriber.workspace = true
 
 [dev-dependencies]
 clap.workspace = true
+tempfile.workspace = true