Filesystem and workspace policy

Import memory filesystem primitives from core's public library. Look at core's OSAccess declaration for the memory namespace handler.

Core owns file operations, open modes, typed filesystem errors and environment values. A new file requires its parent directory to exist. Clock calls may be answered by the worker's own OS policy before reaching a callback. No host filesystem is implicitly exposed by the memory callback.

The private sandbox workspace wraps OSAccess and adds package policy without subclassing core or exposing a new public workspace type. Non-filesystem extension contributions remain composable.

Use SandboxExtension when child execution needs a private workspace. It coordinates child lifecycle, workspace preparation, and collection with the parent OS handler.

Workspace, WorkspaceTransfers, and PersistentTransfer define the workspace, transfer, and native-backing contracts. Choose memory workspaces for browser execution. Choose native backing when the application needs an owned workspace directory.