extract shared mosis-core library from sandbox APIs

Create core/ directory with platform-agnostic sandbox components:
- Timer manager, JSON API, Crypto API, Virtual FS
- Lua sandbox, Permission gate, Audit log, Rate limiter
- Platform abstraction interfaces (IAssetInterface, IFilesystemInterface)
- Platform-agnostic logger with Android/Desktop implementations

Update designer to link against mosis-core library instead of
including sandbox sources directly.

This is the foundation for unifying the Android service and
desktop designer to share the same codebase.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

core/include/mosis/sandbox/path_sandbox.h

@@ -0,0 +1,52 @@
+#pragma once
+
+#include <string>
+#include <filesystem>
+
+struct lua_State;
+
+namespace mosis {
+
+class PathSandbox {
+public:
+    explicit PathSandbox(const std::string& app_path);
+
+    // Validate a path is within the sandbox
+    // Returns true if valid, sets out_canonical to the resolved path
+    bool ValidatePath(const std::string& path, std::string& out_canonical);
+
+    // Check if path contains traversal attempts (..)
+    static bool ContainsTraversal(const std::string& path);
+
+    // Check if path is absolute
+    static bool IsAbsolutePath(const std::string& path);
+
+    // Normalize path separators and remove redundant ./ components
+    static std::string NormalizePath(const std::string& path);
+
+    // Validate module name for require() - alphanumeric, underscore, dots only
+    static bool IsValidModuleName(const std::string& name);
+
+    // Convert module name to relative path (e.g., "ui.button" -> "scripts/ui/button.lua")
+    static std::string ModuleToPath(const std::string& module_name);
+
+    // Get the app's base path
+    const std::string& GetAppPath() const { return m_app_path; }
+
+    // Resolve a relative path to full path within sandbox
+    std::string ResolvePath(const std::string& relative_path);
+
+private:
+    std::string m_app_path;
+};
+
+// Safe require implementation for Lua
+// Loads modules only from app_path/scripts/<module>.lua
+// Caches modules in registry
+int SafeRequire(lua_State* L);
+
+// Register safe require as global "require"
+// The PathSandbox pointer is stored in registry for use by SafeRequire
+void RegisterSafeRequire(lua_State* L, PathSandbox* sandbox);
+
+} // namespace mosis