Featurizer 0.5.2

Advanced-aggregator hardening: full-registry execution coverage (closing the
string-shape-only blind spot), plus the runtime fixes it surfaced.

Added

• Full-registry aggregator execution coverage.
  tests/integration/test_all_aggregators_execution.py now executes every
  registered aggregator on real PostgreSQL over edge-case fixtures (single-row,
  constant, zero/negative, avg-zero, single-category groups; date and
  timestamp temporal columns). Previously only the default-active set had
  execution coverage — the advanced tier was string-shape tested only, which is
  how the v0.5.1 cluster of runtime bugs slipped through. "Every registered
  aggregator executes without error" is now a tested invariant.

Fixed

• harmonic_mean division-by-zero. count(x)/sum(1/x) raised on a zero
  value (1/0) and on a zero denominator. Now positive-domain and guarded:
  case when min(x) > 0 then count(x)/NULLIF(sum(1.0/NULLIF(x,0)),0) else null end
  (NULL on the undefined non-positive domain, mirroring geometric_mean).
• mean_deviation restored as a correct two-pass SubqueryAggregator
  (avg(abs(x - mean)) via a correlated subquery for the mean) and re-added to
  the default set — it had been removed in v0.5.1 because the single-pass form
  nested aggregates. Verified: MAD of [1,4,9,16] = 5.0.
• Planner empty-CTE bug. A single-type aggregation set over a mixed-type
  entity graph (e.g. [entropy] over a numeric-only child) emitted
  select <key>, from … — a dangling comma. The planner now skips emitting the
  aggs CTE (and its join) when an aggregation yields no features for a child.

Removed

• z_score and min_max_scale dropped from the registry. They are per-row
  normalizations, not reductions — their SQL references a bare, un-grouped
  column, invalid in a GROUP BY aggregate. Use the cross_entity_zscore /
  cross_entity_percentile transformers instead. (v0.5.1 had excluded them from
  the default set but kept them registered; they are now fully removed.)