[Performance] Optimize local and remote specifications merging

[gonzaloriestra]

WHY are these changes introduced?

Merging local and remote extension specifications has been performing multiple nested $O(N)$ lookups, leading to $O(N^2)$ time complexity. With dozens of specification configurations, this creates redundant processing inside the CLI startup path.

WHAT is this pull request doing?

Refactors mergeLocalAndRemoteSpecs to use a Map for finding local specifications, and a Set for checking specification identifier presence. This reduces the time complexity of the merge algorithm to $O(N)$, resulting in more efficient and faster startup of CLI commands like app generate extension, app dev, and app deploy.

How to test your changes?

shopify app generate extension

Post-release steps

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

---

PR created automatically by Jules for task 6332725731645540200 started by @gonzaloriestra

[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.