Scripting: export commands a script runs directly respect Allow Scripts to Write Files (#2921)

* Scripting: export commands a script runs directly respect Allow Scripts to Write Files

CompItem.saveFrameToPng and exportAsMotionGraphicsTemplate checked the
file-write preference only in their JavaScript wrappers. A script could call
comp.saveFrameAs or essential.exportTemplate itself (app.run, __exec) and the
native handlers wrote the file with the preference off.

Session::refuse_export and write_output_with now refuse when the running
command was issued by a script and the preference is off, so every command
that writes a path it is given is gated the same way. The wrappers check the
gate with a new side-effect-free __file("checkWrite") instead of writing an
empty file at the destination.

* address review: clear stale export test dir before use

* test: allow script writes in script_imports_footage_and_saves

crates/script/src/runtime.rs

@@ -427,6 +427,8 @@ fn native_file(_: &JsValue, args: &[JsValue], ctx: &mut Context) -> JsResult<JsV
                 let p = p.to_string_lossy();
                 Ok(json!(if op == "exists" { s.services.exists(&p) } else { s.services.is_dir(&p) }))
             }
+            // The write gate alone, with no side effect on the file (export wrappers).
+            "checkWrite" => check_access(s, &path, true).map(|_| json!(true)),
             "read" | "readBinary" => {
                 let p = check_access(s, &path, false)?;
                 let bytes = s.services.read_file(&p.to_string_lossy()).map_err(|e| format!("Unable to read {path}: {e}"))?;