planner/optimizer: CMSWithHeap should move count_events into a subtype #420

planner/optimizer — `CMSWithHeap` currently takes `count_events` as a parameter in `candidate_gen.rs`'s param grid, rather than as a subtype. This is inconsistent with how plain `CountMinSketch` differentiates "sum" vs "count" via `aggregation_sub_type`.

Proposal: remove `count_events` from `CMSWithHeap`'s parameters and instead express it as a subtype, mirroring the CountMinSketch sum/count pattern, for consistency across sketch families.

Found during code review of PR #407.