Optimizer cost model: Subtract cost assumes T = W #765

Problem

cost_model::query_cost prices Subtract as O(1) (merge + subtract + query). That holds only when the repeat interval T equals the window W (design doc open item 6). When T > W, each query must fold in ⌊T/W⌋ newly completed windows and subtract as many evicted ones, so cost scales with T/W.

Once items keep their own T (instead of the gcd), T > W becomes common and Subtract is systematically underpriced.

Change

Price Subtract as a function of T/W, or restrict Subtract candidates to T = W.

Part of #753.

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