Add an upgrade plan: solve for tiers you are about to buy

[OGMods]

Summary

Players asked to solve for an upgrade they are about to buy without having to rebuild afterwards. This adds an upgrade plan: up to three upgrades in buying order. A run solves for the tiers at the end of the plan, and every layout it keeps must also run at today's tiers and after each step. The layout comes back at today's tiers, so what is on screen is what can be built now.

It is optimal only at the last step. Until every upgrade is bought, the layout keeps room for what is coming and makes less power than a plain solve. The dialog says so up front.

Why

Measured over the eight maps (8s solves, two seeds, four progression stages), an upgrade can break a standing layout, and which way depends on the role:

Upgrade	Layout solved for today, after the upgrade	Layout solved for the target, today
Reactor	overheats every time (0–66% of its power left)	runs
Generator	often overheats	often overheats
Cooler	runs, 49–72% of a fresh target solve	overheats every time

A layout solved under a plan never overheated at any step, in any run. It reached 97–100% of the best target power for generator upgrades and ~100% for reactor ones, at 57–98% of today's best power. Cooler upgrades gain nothing: the best layout that also runs today is today's layout. The order matters: requiring every buying order kept 31–59% of the target power, a fixed order 59–99%. So the plan is ordered.

Changes

• Solver (@reactor2/solver): SolveOptions.upgradePlan. Every point where the search accepts a layout as stable also requires it to run at each plan step (best record, repair, composition, pruning, right-sizing, tied layouts, stabilize). The search also builds a starting layout from today's roster; without that, cooler plans came out far worse. resolveUpgradePlan turns the ordered steps into a target plus the rosters along the way, skipping steps already bought.
• Worker path: the resolved plan crosses to the island workers.
• Setup: a row at the top of the Buildings tab opens the plan dialog. Its picker lists only unlocked buildings with a level left to buy (strongest first, with icons). Each step has tier buttons and up/down reordering, and each building card marks its planned tier with a dashed outline.
• Readout: an "After upgrades" line for any board on screen, including solves found without the plan and the player's own board. It names the first step that would overheat the layout. Buildings idle today but needed after the plan are listed as "Kept for upgrades" rather than Idle.
• State: under a plan, runs are compared on power after the plan. Each solve records which plan it was searched under (resultPlanKey), and the plan is kept out of the stored-solve signature, so editing it does not discard other islands' saved solves.
• Docs: docs/SOLVER.md (design note and measurements), the folder CLAUDE.md guides, README.

Testing

• npm test: 854 passing, 25 of them new. They cover the resolver, stability at every step on three plans, evaluatePlan, ranking by power after the plan, and the dialog's plan actions. Without a plan, the same solves overheat, so the step-stability tests have teeth.
• npm run fixtures regenerates byte for byte; the path without a plan is unchanged.
• npm run check and npm run knip are clean.
• Driven in the dev server at desktop and phone width: plan dialog, a run under a plan (1.46T today / 2.18T after the upgrade, nothing overheating), and the warning on a layout solved without the plan (7 buildings overheat at Quantum Reactor Lv. 4).