Editor: CodeLens to scaffold Vega-Lite data transforms

Add a discoverable '+ Add transform' / '+ filter/aggregate/...' CodeLens in the
spec editor that scaffolds field-typed transform[] steps, with a type-to-filter
completion on the same catalog as an accelerator. New pure core
(spec-data-transforms): step-slot and view-site classification, the field-typed
catalog, and comma affixing. Deliberately the one home the schema leaves bare —
inline channel transforms and the transform key stay with the JSON schema.

Rename the composition toolbar menu Transform -> Compose so it reads distinctly
from a data transform. Deferred (recorded at their sites): extract a shared
cursor-lens skeleton across the two installers; a shared completion replace-range
helper (now 3 sites); params scaffolding as the sibling feature.

src/app/components/SpecEditor.tsx

@@ -50,6 +50,10 @@ import {
   configureSpecExpressionHints,
   installExpressionMarkers,
 } from '../services/spec-expression-hints';
+import {
+  configureSpecTransformScaffold,
+  installSpecTransformScaffoldCodeLens,
+} from '../services/spec-transform-scaffold';
 import { runExtract } from '../services/extract-action';
 import { useAppStore } from '../stores/AppStore';
 import { confirm } from '../stores/ConfirmStore';
@@ -173,6 +177,8 @@ configureSpecTransformCodeActions();
 configureSpecDatasetHints();
 // Register the expression completion/signature-help/hover providers once (docs/architecture/08).
 configureSpecExpressionHints();
+// Register the data-transform step-scaffold completion once (docs/architecture/08).
+configureSpecTransformScaffold();
 
 /** The two spec↔config operations, surfaced as an overflow menu (council:
  *  Carbon menu-buttons — overflow for additional options under space
@@ -198,11 +204,12 @@ const CONFIG_ACTIONS = [
 
 type ConfigActionId = (typeof CONFIG_ACTIONS)[number]['value'];
 
-/** Structural transforms, surfaced as a sibling menu to Config (the discoverable
+/** Composition restructures, surfaced as a sibling menu to Config (the discoverable
  *  home; the lightbulb and F1 palette are the accelerators — see
  *  services/spec-transform-actions). They act on the selection, else the whole
- *  spec. */
-const TRANSFORM_ACTIONS = [
+ *  spec. Named "Compose" so it reads distinctly from a Vega-Lite data `transform`
+ *  (the pipeline scaffolded by the editor completion — services/spec-transform-scaffold). */
+const COMPOSE_ACTIONS = [
   { value: 'layer', label: 'Wrap in layer', detail: 'Overlay marks on shared scales' },
   { value: 'hconcat', label: 'Wrap in horizontal concat', detail: 'Place views side by side' },
   { value: 'vconcat', label: 'Wrap in vertical concat', detail: 'Stack views top to bottom' },
@@ -215,7 +222,7 @@ const TRANSFORM_ACTIONS = [
   },
 ] as const;
 
-type TransformActionId = (typeof TRANSFORM_ACTIONS)[number]['value'];
+type ComposeActionId = (typeof COMPOSE_ACTIONS)[number]['value'];
 
 function EditorToolbar({
   editorRef,
@@ -280,7 +287,7 @@ function EditorToolbar({
     else runExtractConfigToTheme(editor);
   };
 
-  const handleTransformAction = (action: TransformActionId) => {
+  const handleComposeAction = (action: ComposeActionId) => {
     const editor = editorRef.current;
     if (!editor) return;
     if (action === 'simplify') runUnwrap(editor);
@@ -344,13 +351,13 @@ function EditorToolbar({
         </Button>
       )}
       <SelectControl
-        id="editor-transform-actions"
-        label="Spec transform actions"
-        heading="Transform"
-        options={TRANSFORM_ACTIONS}
-        onSelect={handleTransformAction}
-        triggerContent="Transform"
-        triggerTitle="Structural transforms — wrap the focused view in a composition, or simplify one"
+        id="editor-compose-actions"
+        label="Spec composition actions"
+        heading="Compose"
+        options={COMPOSE_ACTIONS}
+        onSelect={handleComposeAction}
+        triggerContent="Compose"
+        triggerTitle="Composition restructures — wrap the focused view in a layer/concat/facet/repeat, or simplify one"
         disabled={activeId === null || editorView === 'published'}
       />
       <SelectControl
@@ -447,7 +454,7 @@ export function SpecEditor() {
     const configActionsSub = installSpecConfigActions(editor);
 
     // Structural-transform actions in the F1 palette (the lightbulb is registered
-    // once, globally, above; the toolbar Transform menu is the home). Per editor,
+    // once, globally, above; the toolbar Compose menu is the home). Per editor,
     // disposed below like the config actions.
     const transformActionsSub = installSpecTransformActions(editor);
 
@@ -455,6 +462,10 @@ export function SpecEditor() {
     // editor, because its commands need this editor's handle to apply the edit.
     const codeLensSub = installSpecTransformCodeLens(editor);
 
+    // Cursor-aware data-transform scaffold CodeLens (＋ Add transform / ＋ filter …)
+    // — per editor for the same reason: its commands drive this editor's snippet edit.
+    const scaffoldLensSub = installSpecTransformScaffoldCodeLens(editor);
+
     // Validate Vega expressions in the draft and squiggle the invalid ones — per
     // editor, because it writes markers to this model (docs/architecture/08).
     // Debounced internally; recomputes on edit and on a draft↔published toggle.
@@ -471,6 +482,7 @@ export function SpecEditor() {
       configActionsSub.dispose();
       transformActionsSub.dispose();
       codeLensSub.dispose();
+      scaffoldLensSub.dispose();
       exprMarkersSub.dispose();
       editor.dispose();
       editorRef.current = null;