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/adapters/mock_fs.rs

@@ -24,6 +24,7 @@ impl FileEntry {
 pub struct MockFileSystem {
     files: [Option<FileEntry>; MAX_FILES],
     count: usize,
+    read_only: bool,
 }
 
 impl MockFileSystem {
@@ -31,9 +32,16 @@ impl MockFileSystem {
         Self {
             files: [const { None }; MAX_FILES],
             count: 0,
+            read_only: false,
         }
     }
 
+    /// The same file system, refusing all writes.
+    pub fn read_only(mut self) -> Self {
+        self.read_only = true;
+        self
+    }
+
     fn find_by_name(&self, path: &[u8]) -> Option<usize> {
         for i in 0..MAX_FILES {
             if let Some(entry) = &self.files[i] {
@@ -47,6 +55,10 @@ impl MockFileSystem {
 }
 
 impl FileSystem for MockFileSystem {
+    fn writable(&self) -> bool {
+        !self.read_only
+    }
+
     fn open(&mut self, path: &[u8]) -> Option<FileHandle> {
         let idx = self.find_by_name(path)?;
         if let Some(entry) = &mut self.files[idx] {