Docs: update optimizer-mip-formulation.md for the implemented MILP

[milindsrivastava1997]

Change

Update .design_docs/optimizer-mip-formulation.md to describe the MILP as implemented:

• Assignment unit (AQE, T, accuracy_sla, latency_sla)
• The three window rules (replace FRESHt/FRESHs bounds)
• No EXACT; unservable items are an error
• Label-set facts as inputs
• Accuracy metric table and latency filter
• Linking constraints (z ≤ u, u ≤ Σz)
• Retention depth semantics with shared configs
• HiGHS solver

Coordinate with #652 (doc reconciliation) and fold in .design_docs/milp-port-from-sketch-bench-129.md.

Part of #753.

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