Add Chart Builder: no-JSON Vega-Lite composer from a dataset (M4)

docs/IMPLEMENTATION-PLAN.md

@@ -48,7 +48,7 @@ doc before implementing.
 | **M1**   | **MVP core loop**           | Author a Vega-Lite snippet, see it render live, it persists                    | §02, §03A–C, §04, §09A                      |
 | **M1.5** | Visual design foundation ✅ | Apply the design language: tokens, IBM Plex, restyled M1 surfaces, chart theme | [arch 09](architecture/09-visual-design.md) |
 | **M2**   | Editor robustness           | Draft/Published, validation, schema autocomplete, fit modes                    | §03D–E, §04, §07(editor)                    |
-| **M3**   | Datasets                    | Named reusable data + reference resolution in preview                          | §05, §03F, §09B                             |
+| **M3**   | Datasets ✅                 | Named reusable data + reference resolution in preview                          | §05, §03F, §09B                             |
 | **M4**   | Chart Builder               | No-JSON chart composition from a dataset                                       | §06                                         |
 | **M5**   | Settings + Import/Export    | Preferences + workspace backup/transfer                                        | §07, §08, §09C                              |
 | **M6**   | Shell polish                | Resize/toggle panes, routing, shortcuts, toasts, a11y, offline                 | §01, §10                                    |
@@ -307,11 +307,15 @@ the reference.
 - Build a bar chart from a dataset in a few clicks; preview live-updates;
   Create → new snippet opens and renders.
 
-**Council** — seat **FT Visual Vocabulary** + **Datawrapper** (chart-choice canon) here:
-this is where Astrolabe stops being a pass-through JSON editor and starts making
+**Council** — **FT Visual Vocabulary** + **Datawrapper** are now **seated** (chart-choice
+canon): this is where Astrolabe stops being a pass-through JSON editor and starts making
 chart-shaped suggestions/defaults, so "_which chart, and why_" becomes a decision the app
-owns — the one thing Carbon's data-viz styling doesn't cover. Seating triggers a one-off
-backfill of the builder's defaults/affordances (see [`/council`](../.claude/skills/council/SKILL.md)).
+owns — the one thing Carbon's data-viz styling doesn't cover. Rather than a styling-only
+seating, we ran a full **research-first** pass (FT + Datawrapper + the formal engines
+**Draco** and **Voyager**), recorded in [`docs/chart-builder-research.md`](chart-builder-research.md),
+and chose the **Tier B "smart + guarded"** design: smart default mark for the data shape,
+valid-type-locked field-type menus, Size-channel discipline, and non-blocking guidance.
+The convergent rules and citations live in that doc; the spec (§06) was amended to match.
 
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