CostWeights are a self-consistent stub, not real $/byte-sec vs $/cpu-sec #647

`CostWeights::default()` (cost_model.rs) uses `ingest_mem=1e-9, ingest_cpu=1.0` (and same for query) — a ratio picked to be self-consistent, not derived from real cloud pricing.

Example: two candidates with identical CPU cost but 10x different memory footprint currently differ in total cost by only 9e-9 — real $/GB-month vs $/vCPU-hour would weight that very differently.

Part of #523.