fix(core): consolidate external asset and dependency preservation (#2410)

## Summary

- preserve external SVG fragment references during bundling
- preserve external module scripts and serve `.mjs` with a JavaScript MIME type
- retain template-head stylesheets when mounting sub-compositions
- add regression coverage across compiler runtime and file-server paths

Consolidates and replaces #2390, #2297, and #2375.

## Verification

- core compiler/runtime tests: 89 passed
- producer file-server tests: 48 passed
- core, producer, engine, and CLI typechecks passed
- `git diff --check`

packages/cli/src/utils/compositionServer.ts

@@ -62,6 +62,7 @@ export function injectRuntime(html: string): string {
 
 const ASSET_CONTENT_TYPES: Record<string, string> = {
   js: "application/javascript",
+  mjs: "application/javascript",
   css: "text/css",
   json: "application/json",
   png: "image/png",