Add production-process differential coverage for non-DDSketch families #489

Context

Found while reviewing #478.

The PR documents differential-all as the complete sketch/query differential inventory. In practice:

• promql_differential_process_e2e.rs is production-process + independent raw-value oracle, but DDSketch-only.
• KLL, HLL, CountSketch, Count-Min Sketch, and heap-backed top-k live in e2e_controller_plans_and_backend_serves.rs, which constructs the controller/backend HTTP stack in-process and is not differential against an independent raw-data oracle.
• Eight of those fifteen matrix scenarios currently fail.

The matrix is useful and should remain strict, but it does not yet provide the same process boundary and oracle strength for non-DDSketch families.

Current commands

./scripts/e2e.sh differential
./scripts/e2e.sh differential-all

Expected coverage

Add production-process scenarios for:

• KLL: quantile with an exact rank oracle and justified tolerance
• HLL: cardinality from a deterministic raw distinct set, including multi-SID merge
• CountSketch and Count-Min Sketch: frequency estimates from deterministic keyed events
• heap-backed CountSketch/CMS: top-k labels, ordering, and approximate counts
• instant/range forms where the query language supports both

Each scenario should:

1. ingest through modified OTLP into the production data-plane binary;
2. query only the public PromQL HTTP API;
3. prove ASAP-local execution rather than fallback;
4. compare labels, timestamps, shape, and values with a raw-data oracle;
5. remain executable and non-ignored when it exposes a product regression.

Done when

The all-sketch differential command no longer relies on in-process-only coverage for non-DDSketch families, and its documentation distinguishes stable acceptance scenarios from known failing diagnostics.