Show a bounded native-cache preview before authoritative reconstruction (#463)

Refs #303. file.save / file.save_as can store a warm native geometry cache (cache: true, off by default) keyed to the document hash, app version and adapter revision. Opening shows a bounded preview from it until the rebuilt session replaces it. Any cache failure counts as a miss.

crates/kernel/src/native.rs

@@ -8,6 +8,8 @@ use std::sync::Arc;
 mod adapter;
 mod binary;
 mod metadata;
+mod preview;
+pub use preview::native_preview_mesh;
 
 // 01/02 were development-only prototypes; independent metadata records start at 03.
 const MAGIC: &[u8; 8] = b"SCBREP03";