fix(save): replace files atomically

Replace native engine saves and engine/MCP exports atomically, preserving private permissions during staging and cleaning up failed writes. Add regressions for long filenames and caught writer unwinding.

Fixes #436.

Co-authored-by: aiongg <aiongtaigi@gmail.com>
Co-authored-by: skylar taylor-barrick <hi@skylar.cc>

crates/engine/src/cmd/interchange.rs

@@ -25,7 +25,7 @@ fn export_idml(s: &mut Session, p: &Value) -> Result<Value> {
     match str_param(p, "path") {
         Some(path) => {
             #[cfg(not(target_arch = "wasm32"))]
-            std::fs::write(path, &bytes).map_err(|e| EngineError::Other(format!("{path}: {e}")))?;
+            crate::atomic::write_file(path, &bytes)?;
             Ok(json!({"path": path, "bytes": bytes.len()}))
         }
         None => Ok(json!({"base64": base64_encode(&bytes), "bytes": bytes.len()})),