feat(storage-server): add StorageServer<F: FileSystem> with BlockDevice trait, mock adapters, and tests

crates/servers/storage-server/src/fs/mod.rs

@@ -0,0 +1,3 @@
+// Filesystem implementations go here.
+// Each implements FileSystem using a BlockDevice.
+// e.g., fat16.rs: pub struct Fat16<B: BlockDevice> { ... }