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/cli/src/commands/snapshot.ts

@@ -97,20 +97,9 @@ async function captureSnapshots(
 
   const numFrames = opts.frames ?? 5;
 
-  // 1. Bundle
-  let html = await bundleToSingleHtml(projectDir);
-
-  // Inject local runtime if available.
-  // Uses the same multi-strategy resolver as the studio preview server
-  // (runtimeSource.ts) so snapshot works in dev (tsx), built CLI, and npx.
-  const { loadRuntimeSource } = await import("../server/runtimeSource.js");
-  const runtimeSource = await loadRuntimeSource();
-  if (runtimeSource) {
-    html = html.replace(
-      /<script[^>]*data-hyperframes-preview-runtime[^>]*src="[^"]*"[^>]*><\/script>/,
-      () => `<script data-hyperframes-preview-runtime="1">${runtimeSource}</script>`,
-    );
-  }
+  // 1. Bundle. `bundleToSingleHtml` now inlines the runtime IIFE by default,
+  // so the previous post-bundle runtime substitution is no longer needed.
+  const html = await bundleToSingleHtml(projectDir);
 
   const server = await serveStaticProjectHtml(projectDir, html);