perf: setup rollupOption compatibility via config factory

src/config.ts

@@ -195,20 +195,14 @@ export async function resolveConfig(
       const { main, preload, renderer } = loadResult.config
 
       if (main) {
-        main.build ??= {}
-        setupRollupOptionCompat(main.build)
         userConfig.main = await new MainConfigFactory(main, config, { outDir, root }).build()
       }
 
       if (preload) {
-        preload.build ??= {}
-        setupRollupOptionCompat(preload.build)
         userConfig.preload = await new PreloadConfigFactory(preload, config, { outDir, root }).build()
       }
 
       if (renderer) {
-        renderer.build ??= {}
-        setupRollupOptionCompat(renderer.build)
         userConfig.renderer = await new RendererConfigFactory(renderer, config, { outDir, root }).build()
       }
 
@@ -226,20 +220,15 @@ export async function resolveConfig(
   return resolved
 }
 
-function setupRollupOptionCompat<T extends Pick<ViteBuildOptions, 'rollupOptions' | 'rolldownOptions'>>(
-  buildConfig: T
-): asserts buildConfig is T & {
-  rolldownOptions: Exclude<T['rolldownOptions'], undefined>
-} {
-  buildConfig.rolldownOptions ??= buildConfig.rollupOptions
-}
-
 export abstract class ConfigFactory<T extends MainViteConfig | PreloadViteConfig | RendererViteConfig> {
   constructor(
     protected readonly baseConfig: T,
     protected readonly inlineConfig: InlineConfig,
     protected readonly options: { outDir?: string; root?: string }
-  ) {}
+  ) {
+    baseConfig.build ??= {}
+    baseConfig.build.rolldownOptions ??= baseConfig.build.rollupOptions
+  }
 
   async build(cleanMode?: boolean): Promise<T> {
     /* eslint-disable-next-line @typescript-eslint/no-explicit-any */