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/drivers/block/virtio/src/lib.rs

@@ -76,4 +76,7 @@ impl BlockDevice for VirtioBlockDevice {
     }
 
     fn write_sector(&mut self, _sector: SectorId, _buf: &[u8]) -> bool { false }
+
+    /// Writing is not implemented yet.
+    fn writable(&self) -> bool { false }
 }