fix(precompute): restore weighted sketch updates

[zzylol]

Before

PR #481 treated every raw Prometheus sample sent to bare CMS and CountSketch as one event. That changes metric-value aggregation into scrape counting: samples with values 2, 3, and 5 estimate 3 instead of 10.

ASAPQuery #665 did not make unit weighting unconditional. It introduced an explicit count_events choice between 1.0 and the datapoint value. The current backend physical plan has no equivalent typed count-vs-value contract, so inferring count mode from the sketch kind is incorrect.

After

• Revert the unconditional unit-weight update from fix(precompute): count frequency sketch events #481.
• Bare CMS and CountSketch again add each raw Prometheus sample value.
• Document that event counting requires an explicit typed physical-plan contract.
• Add direct regression tests proving values 2 + 3 + 5 estimate 10 for both sketches.
• Preserve the existing explicit Value/Count modes for heap sketches.

Verification

• cargo fmt -p data_plane -- --check
• cargo test -p data_plane bare_cms_adds_sample_values
• cargo test -p data_plane bare_count_sketch_adds_sample_values

Both targeted tests pass. Existing compiler warnings are unchanged.