fix(carrier): C-04 — document hidden vault capacity limit and add 80% mount warning

src/vfs.rs

@@ -795,6 +795,10 @@ impl SanctumFs {
         Ok(())
     }
 
+    pub fn carrier_manifest_usage(&self) -> Option<(usize, usize, f64)> {
+        self.carrier_fs.as_ref().map(|cfs| cfs.manifest_usage())
+    }
+
     pub fn leak_count(&self) -> u64 {
         self.leak_counter().load(Ordering::Relaxed)
     }