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/app/components/SegmentedControl.module.css

@@ -40,3 +40,11 @@
   background: var(--layer-02);
   color: var(--text);
 }
+
+/* Disabled option: perceivable but inert (dimmed, no hover affordance). */
+.option[aria-disabled='true'],
+.option[aria-disabled='true']:hover {
+  color: var(--text-placeholder);
+  background: var(--bg);
+  cursor: not-allowed;
+}