No cross-AQE sharing — greedy deploys one sketch per AQE even when they could share #650

`greedy_assign` (greedy.rs) assigns each AQE independently to its own cheapest candidate. No facility-location logic exists to let two AQEs share one deployed config. Needs `mip_solver.rs` (good_lp + coin_cbc) per the implementation plan's Phase 3a, plus superset label matching (#649) to make sharing possible at all.

Example: two dashboards both querying `sum_over_time(metric[5m])`, one filtered to `pod=x` and one unfiltered, currently get two separate CMS deployments instead of one shared config with a broader label set.

Part of #523, Phase 3.