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/ports/block-device/src/lib.rs

@@ -18,4 +18,8 @@ pub trait BlockDevice {
     fn sector_count(&self) -> usize;
     fn read_sector(&mut self, sector: SectorId, buf: &mut [u8]) -> bool;
     fn write_sector(&mut self, sector: SectorId, buf: &[u8]) -> bool;
+    /// Whether `write_sector` can succeed at all.
+    fn writable(&self) -> bool {
+        true
+    }
 }