cache power reports

Check version

• I'm running the latest version of Path of Building and I've verified this by checking the changelog

Check for duplicates

• I've checked for duplicate issues by using the search function of the issue tracker

Is your feature request related to a problem?

Power reports are extremely slow to generate, and they are often switched between even without modifying anything.

Describe the solution you'd like

A very simple cache would wok wonders for many use cases. You don't even need to be smart about caching sub-graph, just invalidate on modification. You could also keep a map of the build code to results for when edits are done and reverted often. I do that a lot, but I'm not sure how others use it. These are both pretty low impact changes I would think.

Describe alternatives you've considered

Rewrite in c++ probably isn't a great solution.

Additional context

Also, some progress indicator in a window footer for how much computation still needs to be done would be nice.

The issue does not name files, tests, or an entry point. Start by locating the power-report generation flow and determine how build modifications are detected; then assess caching and the requested progress indicator separately. Done means repeated unchanged reports avoid unnecessary computation and the remaining work is covered by appropriate validation.