⚡ Bolt: Optimize array filtering and short-circuiting for UI state

[acebytes]

💡 What: Replaced an eager .filter followed by .reduce with chained .lazy.filter, and replaced an eager .filter followed by .isEmpty with .contains(where:) in CacheoutViewModel.swift.
🎯 Why: Computed properties like selectedSize and hasSelection are evaluated frequently during UI updates. Eager filtering allocates intermediate arrays which causes unnecessary memory churn, and checking .isEmpty on a filtered array performs an O(N) traversal instead of short-circuiting.
📊 Impact: Reduces intermediate array allocations to zero for these properties, and improves hasSelection to O(1) best-case short-circuit evaluation.
🔬 Measurement: Verify by profiling allocations in Instruments during UI interactions (e.g., selecting/deselecting items), which should show reduced transient array creation.

---

PR created automatically by Jules for task 12094602371660781293 started by @acebytes

[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.