Fixed-dollar arena: the whole stack, one dollar

Budget-run lab (Module 7, L7.5): one shared dollar across an 8-task ladder. Naive (all-premium, no cache, no pacing) dies mid-ladder -- 5/8, $1.0200, BUDGET stop, exit 1. Routed stack (ledger + budget stop + router + locality) finishes 8/8 at $0.4840, exit 0. A 0 percent-cache rerun and a lazy config show how a report can lie. Student config must fit, keep floors, and still fit at 0 percent cache. In-memory only; illustration prices, no model or provider call.

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