planner/optimizer: should SketchProperties be uplevelled to traits / moved to asap_sketchlib? #418

planner/optimizer — `sketch_properties.rs` currently models sketch algebraic properties (mergeable, subtractable, subpopulation-aware) as a plain struct returned by a function, scoped to `asap-planner-rs`.

Architecture question raised in review: should this be uplevelled to traits, and/or should it live in `asap_sketchlib` instead, so other crates that care about sketch algebra (not just the optimizer) can use the same source of truth?

Found during code review of PR #407.