fix(msd): 区分虚拟磁盘挂载能力与文件访问状态

使用原始文件元数据挂载虚拟磁盘，允许主机使用网页文件管理不支持的文件系统。连接期间只读取元数据；初始化和删除持有外层写锁，并同步控制器缓存。

API 的 used/free 改为可空值，新增 file_access 表示 available、unsupported、blocked_while_connected 或 unknown，需要前端配套处理。补充状态序列化、元数据校验和文件系统兼容测试。

src/error.rs

@@ -102,7 +102,9 @@ impl MsdErrorCode {
             Self::MsdDownloadIncomplete => "The remote image download was incomplete.",
             Self::MsdDriveNotInitialized => "The virtual drive is not initialized.",
             Self::MsdDriveConnected => "The virtual drive is connected to the controlled computer.",
-            Self::MsdDriveFilesystemUnsupported => "The virtual drive filesystem is unsupported.",
+            Self::MsdDriveFilesystemUnsupported => {
+                "Web file management does not support this virtual drive format."
+            }
             Self::MsdDriveSizeInvalid => "The virtual drive size is invalid.",
             Self::MsdStorageSpaceUnavailable => {
                 "Available virtual media storage space could not be determined."
@@ -183,7 +185,7 @@ impl MsdErrorCode {
                 "Verify the remote server and network connection, then retry."
             }
             Self::MsdDriveFilesystemUnsupported => {
-                "Reinitialize the virtual drive with a supported filesystem, then retry."
+                "Mount the drive on the controlled computer, or use a supported format for web file management."
             }
             Self::MsdStorageSpaceUnavailable => {
                 "Verify that virtual media storage is available, then retry."
@@ -386,7 +388,7 @@ mod tests {
             (
                 MsdDriveFilesystemUnsupported,
                 "MSD_DRIVE_FILESYSTEM_UNSUPPORTED",
-                "The virtual drive filesystem is unsupported.",
+                "Web file management does not support this virtual drive format.",
             ),
             (
                 MsdDriveSizeInvalid,