Optimizer: window compatibility = lookback%W, T%S, W%S per item #758

Problem

Three versions of the window/freshness rule exist:

• design doc (optimizer-mip-formulation.md FRESHt/FRESHs): W ≤ min T / S ≤ min T (bounds only)
• candidate_gen.rs: S | gcd(W, t_repeat_gcd)
• engine window_compatible: W % S == 0 and range % W == 0, no T check

Change

A config is window-compatible with an item (AQE lookback L, repeat interval T) iff:

1. L % W == 0
2. T % S == 0
3. W % S == 0

(tumbling: S = W). These are exactly sketch-bench#129's is_eligible window rules. Apply per item in candidate generation and eligibility; update the design doc to replace the FRESH bounds.

Part of #753.

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