feat: honest, student-friendly challenges with their own workspace - #110

Challenges used to judge the live numbers on every tick, so every brief flashed Passed in its first quarter second, the verdict flickered, and halving the load slider passed all four. Two of the four lessons were also no longer true.

• A check now decides the verdict: three fixed seeds, a warm-up, then a 45s window at the brief's own load, with your faults dropped and locked fields put back. It runs by itself a second after you stop editing
• Each brief has a slot budget, so adding ten times everything doesn't pass. Shedding, stalled queues, a turned down load and deleting parts all fail
• 13 briefs in 4 levels, from finding the first bottleneck to hot keys and losing a region. Each answer passes on 10 seeds and plausible wrong fixes fail
• New in the engine: crash one instance of a node, which is what makes scale out, not up teachable
• Challenges get their own workspace. Starting one sets your design aside without asking, and leaving brings it back exactly
• The panel is simpler, the copy is written for students, and on a phone the diagram fits above the sheet