fix(precompute): count frequency sketch events

[zzylol]

Before

Bare CMS and CountSketch are the backend realizations for canonical SketchQuery::PointCount, but raw precompute added each datapoint scalar as the sketch weight. Five observations with values [1000, -7, 42.5, 0, 9999] therefore produced a value sum instead of frequency 5. ASAPQuery fixed the CMS count-vs-sum distinction in ProjectASAP/ASAPQuery#665.

After

• Bare CMS and bare CountSketch add exactly one occurrence per accepted observation.
• Value-weighted and count-weighted top-k remain explicit modes of the separate heap updaters.
• Regression tests use non-unit and negative values so the old behavior cannot pass accidentally.

This adapts the upstream fix to the current canonical Planner model: bare frequency sketches have PointCount semantics, while exact/value sums are different summary families.

Verification

• cargo fmt -p data_plane -- --check
• cargo test -p data_plane precompute_engine::accumulator_factory::tests::bare_ (2 passed)
• cargo test -p data_plane precompute_engine:: (259 passed)

The first local build attempt exhausted the worktree filesystem during linking; after removing only regenerated worktree target/ artifacts and using the repository shared target, the commands above passed.