feat(storage): full file protocol over per-client shared region (badged clients, owned handles, clamped lengths, ReadOnly on unwritable disk); init loads via libkos::fs; drop boot shared buffer

crates/servers/storage-server/src/filesystem.rs

@@ -74,4 +74,9 @@ pub trait FileSystem {
     fn delete(&mut self, path: &[u8]) -> bool;
     fn file_size(&mut self, handle: FileHandle) -> Option<usize>;
     fn mkdir(&mut self, path: &[u8]) -> bool;
+    /// Whether anything can be written at all; the server refuses
+    /// writing requests up front when not.
+    fn writable(&self) -> bool {
+        true
+    }
 }