⚡ Bolt: Optimize timestamp parsing with fast path

[benpiper]

• 💡 What: Replaced string splitting with substring indexing for standard HH:MM:SS.mmm timestamps in backend/extractor.py.
• 🎯 Why: To avoid object allocation overhead in frequent string parsing during video rendering.
• 📊 Impact: Roughly 50% speedup for standard timestamp parsing (1.1s vs 2.2s per 1 million ops).
• 🔬 Measurement: Verified using a micro-benchmark directly comparing the original function to the fast path wrapper for 1 million iterations.

---

PR created automatically by Jules for task 11595994782995341402 started by @benpiper

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.