codeSplitter: Dependency doctor: rank the packages you care about to settle conflicts #862

Part of #859.

Let the owner rank the packages they care about most (e.g. ① rwsdk ② react ③ effect), so conflicts are resolved by rule instead of by luck.

• When updates conflict (peer ranges, a breakage that only shows up together), the higher-ranked package keeps its target version and lower ones are held back or re-picked to fit
• A repo-wide ranking for scheduled runs, plus an ephemeral run: "bring these up, solve around them", run once
• Unranked packages fall back to today's behaviour
• UI: a drag-to-rank list with package icons

Open question: how far to solve (peer dependencies only, or try combinations in the test clone with a cap on tries).