Featurizer 0.8.0

Sharding rework: the donorschoose wide config (~36.8k columns) — a backend
crash in every previous snapshot — now materializes live in ~8 minutes, and
every cell of the 3-DB × 3-variant live matrix is green (all-agg is seconds
everywhere). Full refreshed artifacts: specs/live-db-revalidation-v080/
(+ summary page specs/live-db-revalidation-v080.html); decision record:
ADR-0005 amendment.

Changed

• Column-group sharding now clusters columns by dependency lineage.
  _partition_columns buckets the target's output columns by their
  source-CTE signature before bin-packing, so same-lineage columns share a
  group and each companion pre-aggregation CTE is emitted/executed by the few
  groups that need it instead of most of them. Measured on the donorschoose
  wide config (27 groups, ~14.9k columns): max per-group CTE closure
  979 → 287, total closure 11,338 → 2,428, duplicated companion instances
  899 → 18, emitted SQL 29.2 MB → 17.4 MB. Group composition changes
  (which columns share a <stem>_group_NNN table); the feature manifest's
  feature_group column remains the supported mapping, and output column
  names are unchanged (ADR-0007).

• Groups are additionally bounded by a window-function budget
  (max_window_fns_per_group, default 500). PostgreSQL's planning memory
  for N same-spec window functions in one select list is superlinear with a
  hard cliff: measured live, ~675 window columns plan in ~5s while ~1,350
  OOM-killed the backend during a plain EXPLAIN (fresh connection; both
  halves of the same list plan fine — count, not content). The packer closes
  a group early when adding a column would exceed the budget.

  Net effect of the two partitioning changes, measured live on the
  donorschoose wide config (~36.8k output columns, 3,000-row cohort) that
  previously OOM-killed the backend: materializes end-to-end in ~8 minutes
  (32 groups, render 26.5s + execution 461.6s), max group closure 285 CTEs,
  worst per-group EXPLAIN well under 2s.

Fixed

• Sharded re-join no longer collides on carried identifier columns. A
  target that carries relationship keys beyond its id (donorschoose's
  schoolid / teacher_acctid) repeats them in every group query;
  to_dataframe merged groups on (as_of_date, id) only, so pandas raised
  MergeError: duplicate columns at the third group. The materialized path
  now merges on the full GroupedQueries.key_columns tuple.

• Sharded group queries no longer carry dead companion CTEs. Per-group
  reachability now scans the pruned rendering of each target-level agg CTE
  instead of its full-width body, so companion pre-aggregation CTEs whose only
  consumer columns landed in other groups are no longer emitted. PostgreSQL 16
  discards unreferenced CTEs at negligible planning cost (measured), so this
  does not change plan shape — it shrinks the emitted SQL, parse time, and
  render time on wide sharded configs.

Added

• Pre-flight plan-size guardrail. ColumnGroupSharder.plan_size_report()
  maps each column group to its live CTE-closure size, and warn_plan_size()
  (wired into every grouped path) logs one loud, actionable warning when any
  group's closure predicts a PostgreSQL planner blowup — the failure mode
  diagnosed on the donorschoose wide config, where ~1000-CTE group queries
  took 30–45s of planning each and OOM-killed the backend during a plain
  EXPLAIN. The warning names the worst groups and the config levers
  (transformers / intervals / entities) instead of letting the run die
  minutes later with "server closed the connection unexpectedly".