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/spec/06-chart-builder.md

@@ -33,12 +33,30 @@ A two-pane modal:
   - Optionally overrides the channel's **field type**. The override appears only once a column is selected, and offers only the **types valid for that column** (Tier B valid-type locking) — a string/boolean column never offers Quantitative, and only a date column offers Temporal. Concretely: number → {Quantitative (default), Ordinal, Nominal}; date → {Temporal}; text → {Nominal (default), Ordinal}; boolean → {Nominal}. When a column admits only one valid type, no override control is shown.
 - When a column is chosen, its field type defaults from the dataset's inferred column type (numeric → Quantitative, date → Temporal, otherwise Nominal); the user may change it within the valid set above.
 - **Size discipline:** the **Size** channel accepts only Quantitative or Ordinal columns — size implies an ordered magnitude, so categorical (Nominal) and Temporal columns are not offered for Size (they remain available on X/Y/Color). A column that can't go on Size is shown disabled there with a brief reason.
+- The column dropdown also offers a field-less **"Count of records"** measure (Vega-Lite `count`) — a quantitative count of the rows, with no column.
 - Clearing a channel back to "None" leaves it out of the produced spec.
 - A **Swap X/Y** control exchanges the X and Y mappings (field and type) in one click, for quickly flipping the axes of the pre-populated default without re-selecting both columns.
 
+The field-type override is presented as a fixed **`N | O | Q | T`** segmented control (abbreviations with full-name tooltips, after _Datasets_' Nominal/Ordinal/Quantitative/Temporal), always showing all four with the column's invalid types **disabled** rather than hidden — so the control keeps one shape on every channel.
+
+### Transforms (per channel)
+
+Once a column is mapped, the channel offers the transforms that apply to its field type — and only those:
+
+- **Aggregate** (a measure / Quantitative field): one of `Sum`, `Mean`, `Median`, `Min`, `Max`, or `None`. (The field-less `Count` measure is chosen via the "Count of records" column option above.)
+- **Bin** (a Quantitative field): bins the values into ranges — e.g. a Quantitative X binned with a Count Y is a histogram. Binning and aggregating the same field are mutually exclusive (setting one clears the other).
+- **Granularity** (a Temporal field): a Vega-Lite `timeUnit` — Year, Year-Quarter, Year-Month, Year-Month-Day, Quarter, Month, Week, Day of month, Day of week, Hour — or `None` (raw timestamps). Defaults to **None** (no silent change to what the raw data shows).
+
+### Sort and stacking (chart-level)
+
+These controls appear only when they apply:
+
+- **Sort** (when X and Y form a category-vs-measure pair): sorts the categorical axis by the measure — `Ascending`, `Descending`, or `None` — the standard way to rank a bar chart.
+- **Stacking** (a Bar or Area mark with a Color series): `Stacked` (absolute) or `100%` (normalized, part-to-whole). Bars/areas without a Color series, or other marks, show no stacking control.
+
 ### Default pre-population
 
-- On open, the first detected column is assigned to **X** and the second (if any) to **Y**, each with its derived field type. Remaining channels start unmapped. The mark starts at the smart default for that X/Y shape (see _Mark type_), not unconditionally Bar.
+- On open, the first detected column is assigned to **X** and the second (if any) to **Y**, each with its derived field type and no transforms. Remaining channels start unmapped. The mark starts at the smart default for that X/Y shape (see _Mark type_), not unconditionally Bar.
 
 ### Guidance (non-blocking)
 
@@ -48,6 +66,7 @@ The builder surfaces short, plain-language hints for configurations that render
 - A **Bar/Line/Area** whose X and Y are both categories (nothing to measure).
 - **Two measures** on a non-scatter mark (a scatter — Point/Circle — usually reads better).
 - An **Area** chart split into multiple colour series (per-series change is hard to see).
+- A **Bar/Line/Area** that pairs a category axis with a **raw (un-aggregated) measure** over a many-row dataset — it draws one mark, and one axis label, per row, so the category axis becomes an unreadable picket fence. The hint suggests aggregating the measure (one mark per category) or, for a bar, flipping to a horizontal bar (Swap X/Y) where long labels stay readable (FT Visual Vocabulary / Datawrapper). Only the un-aggregated case (mark-count = row-count) is detected; flagging an _aggregated_ axis that still has many distinct categories needs per-column distinct counts the profiler does not yet compute (a known gap).
 
 A clean configuration shows no hints.
 
@@ -72,12 +91,11 @@ A clean configuration shows no hints.
 
 Selecting "Create Snippet" produces the final artifact:
 
-- Builds a complete Vega-Lite spec containing: the schema reference, a named data reference to the dataset, the chosen mark (with tooltips enabled), the mapped encodings (each with its field and field type), and any explicit width/height.
+- Builds a complete Vega-Lite spec containing: the schema reference, a named data reference to the dataset, the chosen mark (with tooltips enabled), the mapped encodings (each with its field and field type, plus any aggregate / bin / `timeUnit` transform), chart-level sort and stacking where set, and any explicit width/height.
 - Channels left unmapped are omitted; if no encodings exist the spec omits the encoding block entirely (prevented by validation here).
 - Creates a new snippet from that spec with an auto-generated descriptive name, adds it to the snippet library, and records that it was built from the dataset.
 - Links the snippet to the dataset by recording the dataset reference, so the bidirectional snippet↔dataset relationship is established (see _Datasets_).
-- Raises a success toast naming the created snippet.
-- Closes the builder; the newly created snippet becomes the active snippet in the library/editor.
+- Closes the builder; the newly created snippet becomes the active snippet in the library/editor. **No success toast** — the result is immediately visible (the new snippet opens in the editor), so a toast would be noise (architecture 10 §1, "toast only what the user can't already see"). This refines the earlier blanket "every action toasts" rule, consistent with the Extract-to-dataset / publish reconciliation.
 
 ## Closing