fix(bundler): runtime mode opt-in, ASI-safe joinJsChunks, prune dead subs

Per @vai-bot's review on hf#641:

Important #1: dead `src=""` substitution sites
=============================================

Now that `bundleToSingleHtml` inlines the runtime IIFE by default, the empty
`src=""` placeholder is never emitted in the no-env-var path — the 5 downstream
substitution sites that grep for `src=""` were dead.

Two of them (studio dev server + studio vite preview) genuinely WANT the
placeholder so they can hot-reload a local /api/runtime.js endpoint without
re-inlining ~150 KB on every composition edit. Three of them (CLI validate,
snapshot, layout) were just doing the same inlining the bundler already does.

Resolution:
- Add a `runtime: "inline" | "placeholder"` option to `BundleOptions`. Default
  is "inline" (matches the self-contained-bundle promise the function name
  makes). The two studio surfaces explicitly pass `{ runtime: "placeholder" }`
  to opt in.
- studioServer.ts + studio/vite.config.ts: pass the option, keep their
  existing string-replace logic unchanged.
- validate.ts + snapshot.ts + layout.ts: delete the now-redundant runtime
  substitution code (regex never matches the new inlined-runtime shape).

Important #2: joinJsChunks ASI hazard
======================================

The new helper appended `;` to chunks not already ending in `;` and joined
on `\n`. If a chunk ended with a `// line comment`, the appended semicolon
was eaten by the comment, leaving the next chunk's first statement attached
to the previous chunk's last expression — exactly the ASI hazard the helper
exists to prevent.

Fix: append `\n;` instead of `;` for chunks not already terminated. The
newline closes the line comment, the standalone `;` becomes the statement
separator. For typical chunks (already ending in `;`), output is unchanged
— still clean `\n`-joined chunks with no bare-semicolon lines.

Also added a trailing `;` to `wrapScopedCompositionScript`'s IIFE close
(`})()` → `})();`) so composition scripts join cleanly without falling
through to the `\n;` fallback.

New test: regression guard at the chunk boundary verifies every inline
script body in the bundle parses cleanly via esbuild even when a source JS
file ends with a line comment.

Verification
============

- `bun run --filter @hyperframes/core test` — 653/653 pass
- `bun run --filter @hyperframes/cli test` — 243/243 pass
- `bun run --filter @hyperframes/{core,cli,studio} typecheck` — clean
- `bunx oxfmt --check` + `bunx oxlint` on all touched files — clean

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/studio/vite.config.ts

@@ -60,7 +60,9 @@ interface ScreenshotClip {
 
 function createViteAdapter(dataDir: string, server: ViteDevServer): StudioApiAdapter {
   // Lazy-load the bundler via Vite's SSR module loader
-  let _bundler: ((dir: string) => Promise<string>) | null = null;
+  let _bundler:
+    | ((dir: string, options?: { runtime?: "inline" | "placeholder" }) => Promise<string>)
+    | null = null;
   let _producerModulePromise: Promise<{
     createRenderJob: (config: {
       fps: 24 | 30 | 60;
@@ -78,7 +80,7 @@ function createViteAdapter(dataDir: string, server: ViteDevServer): StudioApiAda
     if (!_bundler) {
       try {
         const mod = await server.ssrLoadModule("@hyperframes/core/compiler");
-        _bundler = (dir: string) => mod.bundleToSingleHtml(dir);
+        _bundler = (dir, options) => mod.bundleToSingleHtml(dir, options);
       } catch (err) {
         console.warn("[Studio] Failed to load compiler, previews will use raw HTML:", err);
         _bundler = null as never;
@@ -171,8 +173,10 @@ function createViteAdapter(dataDir: string, server: ViteDevServer): StudioApiAda
     async bundle(dir: string) {
       const bundler = await getBundler();
       if (!bundler) return null;
-      let html = await bundler(dir);
-      // Fix empty runtime src from bundler — point to the CDN runtime
+      // Studio vite preview: bundler emits an empty `src=""` placeholder so we
+      // can point it at the local /api/runtime.js endpoint. Cached by the browser
+      // across composition hot-reloads instead of being inlined fresh each time.
+      let html = await bundler(dir, { runtime: "placeholder" });
       html = html.replace(
         'data-hyperframes-preview-runtime="1" src=""',
         `data-hyperframes-preview-runtime="1" src="${this.runtimeUrl}"`,