Chart Builder: open a snippet in the builder to edit it in place

docs/spec/06-chart-builder.md

@@ -164,6 +164,16 @@ Selecting "Create Snippet" produces the final artifact:
 - Links the snippet to the dataset by recording the dataset reference, so the bidirectional snippet↔dataset relationship is established (see _Datasets_).
 - 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.
 
+## Open in builder (edit in place)
+
+The builder is also the way to **revise** a chart it could have produced — not only create one. The **editor toolbar** (top of the Spec Editor, alongside the Draft/Published toggle, Config, Revert, and Publish) offers an **Open in builder** action that reopens the active snippet in the builder, populated from its spec — the visual counterpart to editing the same chart's JSON, placed where that editing happens.
+
+- **When it is offered.** Only when the active snippet's **published spec** is **losslessly representable** in the builder's dialect _and_ the dataset it references still exists. Losslessness is judged by parsing the spec back to a builder configuration and **re-assembling it for an exact comparison** against the original (ignoring key order, the `$schema` stamp, and the builder's injected `tooltip`), not by enumerating supported features — so the gate stays correct automatically as the builder's dialect grows. A spec the builder cannot reproduce exactly (hand-authored richness, an unsupported channel/mark, inline `data.values`, a `url` source) stays **Monaco-only**, and the action is **hidden** for it — the same content-gated treatment as _Extract to Dataset_ (a permanently-disabled control the user can't enable in the moment would read as broken; see _Interaction & Feedback_ → action visibility). The builder references a dataset **by name** — its sole data model — so only a snippet that references a saved dataset can hydrate.
+- **What opening does.** Hydrates the builder from the snippet — mark, encodings, transforms, sort/stack, title/subtitle, size — and loads the referenced dataset's columns. The configuration pane names the snippet under edit ("Editing _name_"), and the dataset picker behaves as a **rebase** (built-on work) for any subsequent dataset switch — a loaded chart is never treated as a fresh default.
+- **An edit session is builder-local, not part of the URL.** Like the builder's in-progress configuration generally, the "editing _name_" context is transient: a reload, Back, or shared link reopens the builder as a fresh **create** flow on the same dataset rather than restoring the edit. No data is lost — the published snippet is untouched until _Save changes_ — and re-entering the edit is one click from the toolbar.
+- **Saving.** The primary action becomes **Save changes** (in place of "Create Snippet"): it **republishes** the built spec into the same snippet — overwriting **both** its published and draft versions, so there is no pending draft to reconcile — while keeping the snippet's identity, timestamps' `created`, and dataset links. A user-chosen name is preserved; an auto-named snippet re-derives its name from the new content (as _Publish_ does). The edited snippet becomes the active snippet. As with Create, there is no success toast (the result is immediately visible) and the action is gated on the same validation (at least one channel bound).
+- **The JSON stays the document.** Because Open-in-builder is strict, the builder never silently overwrites a richer spec it cannot represent — it is a view that emits the spec, never a competing source of truth.
+
 ## Closing
 
 - The builder can be dismissed without creating anything (close control / modal dismissal).