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/init.ts

@@ -2042,6 +2042,7 @@ export function initSandboxRuntimeModular(): void {
     const compositionLoaderParams = {
       injectedStyles: state.injectedCompStyles,
       injectedScripts: state.injectedCompScripts,
+      injectedLinks: state.injectedCompLinks,
       parseDimensionPx,
       onDiagnostic: ({
         code,
@@ -3047,12 +3048,21 @@ export function initSandboxRuntimeModular(): void {
       }
     }
     state.injectedCompStyles = [];
+    for (const linkEl of state.injectedCompLinks) {
+      try {
+        linkEl.remove();
+      } catch (err) {
+        // ignore cleanup failures
+        swallow("runtime.init.site15", err);
+      }
+    }
+    state.injectedCompLinks = [];
     for (const scriptEl of state.injectedCompScripts) {
       try {
         scriptEl.remove();
       } catch (err) {
         // ignore cleanup failures
-        swallow("runtime.init.site15", err);
+        swallow("runtime.init.site16", err);
       }
     }
     state.injectedCompScripts = [];