controller: planner picks sketch family per metric from query intent (5-sketch demo)

[zzylol]

Summary

Pin the (sketch, statistic) capability matrix as a first-class module
in controller/src/sketch_algebra/capability_matching.rs and route
bind_workload_typed through it. The MVP demo contract (issue #46)
requires per-metric family selection that the priority-based dispatcher
could not produce — DDSketch (priority 6) always beat KLL (priority 5)
on Quantile, so the rank-err preference for byte-size metrics was
unreachable.

Family-per-statistic mapping (issue #46)

metric	family	query class
http_requests_total	raw passthrough	sum / rate / count
http_latency_ms	DDSketch	quantile (rel-err)
request_size_bytes	KLL	quantile (rank-err)
unique_users_per_min	HLL	cardinality
top_endpoint_qps	CountSketch	top-K
endpoint_request_freq	CountMinSketch (CMS)	frequency

Capability matrix (final state)

sketch	Quantile	Cardinality	TopK	Frequency	SumRateCount
DDSketch	yes	no	no	no	no
KLL	yes	no	no	no	no
HLL	no	yes	no	no	no
CountSketch	no	no	yes	no	no
CMS	no	no	no	yes	no

SumRateCount has no valid sketch — the agent emits raw OTLP and the
backend computes the answer directly.

sketch_family_override behaviour

The workload-spec's sketch_type_override (= the spec's
sketch_family_override per orchestrator contract) wins over the
capability-matched pick when the override is valid for the statistic
class. An invalid override (HLL on a Quantile workload) is silently
dropped and the planner falls back to the catalog default — the
binding never produces a nonsense (sketch, stat) pair.

Files touched

• controller/src/sketch_algebra/capability_matching.rs (new) —
  StatisticClass, AccuracyPreference, is_valid_pair,
  pick_family, classify_demo_metric. 14 unit tests pin the matrix.
• controller/src/sketch_algebra/params.rs — add
  From<SketchType> for SketchKind for the override translation.
• controller/src/sketch_algebra/mod.rs — register module + re-exports.
• controller/src/planner/rules.rs::bind_workload_typed — capability-
  matrix-driven family pick; family-specific Bind* rule dispatch
  (bypasses the priority-based dispatcher's tie-break); 10 unit tests
  covering every contract row, override behaviour, and invalid-override
  fallback.

Test plan

• All 14 capability-matching unit tests pass (cargo test --release sketch_algebra::capability_matching)
• All 10 planner family-per-metric tests pass (cargo test --release planner::rules::tests::typed_binding_*, *sketch_type_override*, all_six_contract_metrics_produce_expected_family)
• All 6 contract metrics produce expected SketchExpr family (covered by the all_six_contract_metrics_produce_expected_family sweep test)
• All 56 sketch_algebra tests pass; pre-existing planner::cost_model::workload_cost_tests::* failures (unresolved Ref("w")) are unrelated to this PR and reproduce on origin/main
• cargo build --release clean (warnings only)

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