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/core/src/runtime/state.ts

@@ -77,6 +77,7 @@ export type RuntimeState = {
   beforeUnloadHandler: (() => void) | null;
   injectedCompStyles: HTMLStyleElement[];
   injectedCompScripts: HTMLScriptElement[];
+  injectedCompLinks: HTMLLinkElement[];
   cachedTimedMediaEls: Array<HTMLVideoElement | HTMLAudioElement>;
   cachedMediaClips: RuntimeMediaClip[];
   cachedVideoClips: RuntimeMediaClip[];
@@ -116,6 +117,7 @@ export function createRuntimeState(): RuntimeState {
     beforeUnloadHandler: null,
     injectedCompStyles: [],
     injectedCompScripts: [],
+    injectedCompLinks: [],
     cachedTimedMediaEls: [],
     cachedMediaClips: [],
     cachedVideoClips: [],