Chart builder: discoverable entry points — Build Chart door, dataset picker, no-datasets state

docs/spec/06-chart-builder.md

@@ -6,15 +6,30 @@ The Chart Builder is a visual, no-JSON way to compose a Vega-Lite chart from a s
 
 ## Opening
 
-- Launched from a selected dataset in the _Datasets_ manager via that dataset's "build chart" action.
-- Opens as a modal dialog over the application; the URL reflects the dataset's "build" action so the open builder is shareable/restorable (see _Application Shell & Navigation_).
-- On open, the builder loads the selected dataset, displays its name, and pre-populates sensible defaults (see below). If no dataset is available, it shows a "No dataset loaded" message and offers no controls.
+The builder has several doors; the guided creation path must be visible where the intent to make a chart forms, not buried behind the data model.
+
+- **The library's creation surface** — the primary **Build Chart** action (see _Snippet Library → The List_). Opens the builder with no preselected dataset; the builder picks the **most recently modified dataset** itself.
+- **The onboarding canvas** — the data-first "Build a chart from your data" door (see _Snippet Library → First-Run & Empty Workspace_).
+- **A dataset's "Build Chart" action** in the _Datasets_ manager — the contextual shortcut; opens the builder on that dataset.
+- Opens as a modal dialog over the application; the URL reflects the loaded dataset's "build" form (or `#build` while no dataset is loaded) so the open builder is shareable/restorable (see _Application Shell & Navigation_).
+- On open, the builder loads its dataset and pre-populates sensible defaults (see below).
+
+### Dataset picker
+
+Which data the chart builds from is itself a builder choice. A **Dataset** picker sits at the top of the configuration pane, showing the loaded dataset and letting the user switch to any other without leaving the builder.
+
+- Switching while the configuration is still the untouched opening default **re-derives fresh smart defaults** for the new dataset.
+- Switching after the user has built on the configuration **rebases** it instead: chart-level intent survives (mark, title/subtitle, explicit size, sort/stack, calculated fields, expression filters), while bindings to columns the new dataset lacks are shed (encodings cleared, predicate filters dropped). A same-schema dataset — the common switch — keeps everything.
+
+### No datasets
+
+With an empty dataset library the builder shows a **no-datasets state** instead of controls: it says what the builder does and offers one primary next step — **Add a dataset**, which opens the _Datasets_ manager on its create form. The guided path never dead-ends.
 
 ## Layout
 
 A two-pane modal:
 
-- **Left — configuration:** dataset name, a **Data** section (row filters, calculated fields, and a collapsible row preview — see _Data_ below), mark type selector, a **field shelf** (the dataset's columns, field-first), a **Marks** card (the Colour and Size encodings), and a "Create Snippet" action. Chart-level properties (title/subtitle, width/height) sit on the **preview side**, under the chart (see _Chart properties_).
+- **Left — configuration:** the dataset picker, a **Data** section (row filters, calculated fields, and a collapsible row preview — see _Data_ below), mark type selector, a **field shelf** (the dataset's columns, field-first), a **Marks** card (the Colour and Size encodings), and a "Create Snippet" action. Chart-level properties (title/subtitle, width/height) sit on the **preview side**, under the chart (see _Chart properties_).
 - **Right — live preview:** the **Columns** (X) and **Rows** (Y) shelves stacked above a rendered chart that updates as the configuration changes, with a placeholder/error area. Position is a property of the chart, so its controls sit on the chart (Tableau's Columns/Rows metaphor).
 
 ## Data (preview, filters, calculated fields)