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.

docs/architecture/05-rendering-theming-preview.md

@@ -352,6 +352,18 @@ blank mid-edit.
 - **Don't** render synchronously on every keystroke.
 - **Don't** await a render inside an input/keydown handler.
 
+### A second preview surface: the Chart Builder
+
+The editor's `LivePreview` is **bound to the snippet editor** — it reads `SnippetStore`
+(shown spec), `AppStore` (fit mode/theme), and `PreviewStore` (shared error). The
+**Chart Builder modal** needs a preview of a _different_ spec source (its config), so it
+does **not** reuse `LivePreview`; it runs its own small debounced render over the same
+`chart-renderer.renderSpec` + `prepareSpecForRender`, with **local** error state (never the
+shared `PreviewStore`, which would cross-talk with the editor). Two preview surfaces, one
+renderer service. Builder flow: `chart-builder.ts` (pure spec assembler) → `ChartBuilderStore`
+(config + create) → `ChartBuilderModal`'s `BuilderPreview`. Reach for a reusable preview
+component only if a _third_ surface appears.
+
 ---
 
 ## 6. Rendering Contract Lives Upstream (reference)