Add aggregation, binning, granularity, sort, and stacking to the Chart Builder

- Per-channel transforms: aggregate (sum/mean/median/min/max), quantitative
  bin, and temporal timeUnit granularity; bin and aggregate are mutually
  exclusive. A field-less "Count of records" measure (Voyager's count(*)).
- Chart-level sort (rank a categorical axis by its measure) and stacking
  (zero / 100% normalize), each shown only when it applies.
- Field type is a fixed N|O|Q|T segmented control with the column's invalid
  types disabled; SegmentedControl gains APG-correct disabled options.
- A crowded-category-axis warning (a raw measure drawing one mark per row over
  a large dataset) and a disabled-Create hint (says why it's disabled).
- Drop the Create success toast — the new snippet is immediately visible.
- Docs: spec §06, research-doc §8 backlog (incl. the cardinality/extent
  profiling TODO), architecture 01 (stable-selector rule) and 05 (builder-local
  preview), and a profiling breadcrumb.

src/core/profile.ts

@@ -84,6 +84,10 @@ export function profileData(
   if (columns.length === 0) return naProfile(size);
 
   const sample = sampleRows(rows);
+  // TODO (backlog: docs/chart-builder-research.md §8 — A3/A4 enabler): in this same
+  // sample pass, also derive a capped per-column distinct count (cardinality, cap ~50)
+  // and numeric extent (min/max → sign), surfaced on DatasetProfile, to power the Chart
+  // Builder's crowded-legend / high-cardinality warnings and its negative-value Size guard.
   const columnTypes = columns.map((name) => ({
     name,
     type: inferColumnType(sample.map((r) => r[name])),