Make compaction transitions recoverable

src/engine/metal/checkpoint.rs

@@ -81,6 +81,8 @@ impl Executor {
         write_u64(file, self.model.main.len())?;
         write_u64(file, self.model_modified.0)?;
         write_u32(file, self.model_modified.1)?;
+        file.write_all(&self.model_identity)
+            .map_err(|error| error.to_string())?;
         for weight in [self.weights.token_embedding, self.weights.output] {
             write_u64(file, weight.offset)?;
             write_u64(file, weight.bytes)?;
@@ -185,6 +187,12 @@ impl Executor {
         if read_u64(file)? != self.model_modified.0 || read_u32(file)? != self.model_modified.1 {
             return Err("KV checkpoint model file has changed".into());
         }
+        let mut model_identity = [0; 32];
+        file.read_exact(&mut model_identity)
+            .map_err(|error| error.to_string())?;
+        if model_identity != self.model_identity {
+            return Err("KV checkpoint model identity or quantization changed".into());
+        }
         for weight in [self.weights.token_embedding, self.weights.output] {
             if read_u64(file)? != weight.offset
                 || read_u64(file)? != weight.bytes