Track build output size and fail CI if it increases too much

I've seen libraries do this, where they have a file saying a max allowed bundle size, and you have to increase it manually when the bundle gets bigger. This makes sure there are no accidental large bundle size increases like the one I caught in https://github.com/oxidecomputer/console/pull/2910#issuecomment-3403530384. Unlike some libraries, we expect our total size to be going up regularly as we add more pages and features. That could just mean we increment the max in bigger chunks rather than incrementing it the minimum possible to let the test pass. We could also treat the root chunk separately since it's the thing that users load immediately.

Start by locating the build command and CI configuration that produce and validate the bundle size. Define a committed size limit, decide whether the root chunk is tracked separately, and verify that normal feature growth requires deliberate threshold updates while unintended increases fail CI.