Asset database file ops + script-file creation helpers

Groundwork for the Stage-10 editor-UX batch (file explorer, New Script
button), all pure logic proven by unit tests:

- `AssetDatabase::move_asset` / `move_folder` / `delete_asset`: perform
  the disk operation and the uid-map update together, so renaming or
  moving an asset (or a whole folder) keeps its uid and every saved
  `AssetRef` resolving. Kinds re-classify from the new path; `..` and
  existing destinations are refused via the new `AssetDbError` enum;
  deleted uids are never reused.
- Editor `assets.rs`: `script_template` (compiles under the sandboxed
  ScriptEngine — covered by a test), `sanitize_script_stem`, and
  `create_script_file` which writes a collision-free
  `assets/scripts/<stem>.rhai` and returns the relative path for
  database registration.
- docs/assets.md: new "File operations" section + the missing Script
  row in the typed-folder table.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

engine/src/asset/mod.rs

@@ -18,7 +18,8 @@ mod handle;
 mod server;
 
 pub use database::{
-    asset_ref_target, AssetDatabase, AssetEntry, AssetKind, AssetRef, AssetUid, ASSET_MANIFEST_FILE,
+    asset_ref_target, AssetDatabase, AssetDbError, AssetEntry, AssetKind, AssetRef, AssetUid,
+    ASSET_MANIFEST_FILE,
 };
 pub use gltf::{load_gltf, load_gltf_slice, GltfError, GltfLoader, GltfMesh, GltfModel};
 pub use handle::{AssetId, Handle, LoadState};