Add Concurrency Support for Placeholder Resolvers

[egyjs]

Context
Currently, placeholders in ProgressiveJsonStreamer are resolved sequentially, which means any slow DB/API call blocks the stream. This works fine for small payloads but limits scalability and responsiveness in larger real-time APIs or dashboards.

---

💡 Proposal: Concurrency with amphp/parallel

We're considering supporting concurrent resolution of placeholder values using:

• ✅ amphp/parallel to run blocking placeholder resolvers in parallel workers
• ✅ Optional usage — maintain backward compatibility

---

🔍 Goals

• Resolve multiple placeholders in parallel
• Stream results as soon as they’re ready
• Avoid blocking early critical data (e.g. header, meta) on slow sections like comments or analytics

📦 Example API

$streamer->concurrent(); // Enables concurrency

$streamer->addPlaceholders([
    'header' => fn() => fetchHeader(), // fast
    'post' => fn() => slowPostFromDB(), // slow
    'comments' => fn() => fetchCommentsFromApi(), // slower
]);

---

✅ Why this matters

• More responsive user experiences
• Ideal for Suspense-based frontends (React/Vue)
• Scales better in high-latency data scenarios

---

🗣️ Feedback

• Would you use this?

• Would you want control over concurrency (e.g. max workers, timeout)?

• Should this be the default or opt-in?