Optimizer: accept externally provided label-set facts (series count, cardinality) #756

Problem

The optimizer derives N_g from --dataset (CSV series inventory) and uses one global placeholder arrival rate --rho (#563). Subpopulation-aware sketches are costed with a constant SUBPOPULATION_COUNT. Memory for Multiple* accumulators (unbounded hashmaps) is costed as one flat-stub instance regardless of group count, so they always beat their per-group twins.

Change

Accept externally provided label-set facts from a file (--label-set-facts <path>, required). The optimizer does not estimate them. Removes --dataset, --rho, SeriesDataset (dataset.rs), and SUBPOPULATION_COUNT.

Inputs

• Metric schema comes from the workload YAML's metrics: hints (ControllerConfig::schema_from_hints), same as asap-planner. Error if metrics: is absent or any workload metric is missing from it.
• Facts file (YAML), two tables:

  series:                         # per (metric, spatial_filter)
    - metric: http_requests_total
      spatial_filter: 'job="api"'   # required; "" = unfiltered
      series_count: 1000
  groups:                         # per (metric, spatial_filter, grouping_labels)
    - metric: http_requests_total
      spatial_filter: 'job="api"'
      grouping_labels: [service]    # required; resolved `by` labels (also for `without` queries)
      cardinality: 5

• Arrival rate is derived, not provided: λ(metric, filter) = series_count × 1000 / scrape_interval_ms (scrape interval = --data-ingestion-interval-ms). Assumes one sample per series per scrape; overestimates sparse/irregular series.

Matching and validation

• Exact-key match only; no inference across label sets.
• spatial_filter is normalized with asap_types::normalize_spatial_filter (same as the workload side). Missing-key errors print the exact normalized key expected.
• Missing series or groups facts for any item → error listing all missing keys.
• 1 ≤ cardinality ≤ series_count; grouping_labels: [] requires cardinality == 1; a groups row without a series row → error; duplicate keys → error.
• Facts matching no item → warn (one file may serve several workloads).

Cost model

Cardinality card enters per sketch class (one label set per candidate; no inner/outer key split):

Merge/subtract CPU follow the memory multiplier.

Analytical per-key memory for the trivial accumulators, applied to both per-group (Sum/MinMax/Increase) and Multiple* twins so each pair ties on memory:

mem_per_key = (n_grouping_labels × 4 + value_bytes) × 8/7
value_bytes: Sum/MinMax = 8, Increase = 72

Label values assumed dictionary-encoded (u32 codes, dictionary amortized and not charged). Ignores Increase counter-reset events. Underestimates Multiple* in today's engine, whose inner maps hold Vec<String> keys. KLL/HLL/CMS keep measured/stub costs (#524); Set/DeltaSet stay on the stub (#763).

Out of scope

• Facts-file generator (asap-tools); add when a real caller (e.g. feat(asap-tools): label query sets and skew fits for cluster traces #746/feat(asap-tools): validate recommended sketch configs end to end on cluster traces #752 cluster traces) drives the optimizer.
• Semantic (parsed) spatial-filter matching (normalize_spatial_filter TODO).

Deliverables

• Inline YAML fixtures in tests; update .design_docs/optimizer-v1-implementation-plan.md CLI examples; one example facts file next to the doc.

Refs #693, #563, #525, #524.

Part of #753.

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