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.

docs/architecture/08-vega-editor-techniques.md

@@ -285,7 +285,15 @@ logic is pure `src/core/`; the Monaco glue is thin app-layer services.
 **Core (pure, portable):**
 
 - `spec-transforms` — wrap a view in `layer`/`hconcat`/`vconcat`/`facet`/`repeat`; collapse
-  a single-child composition (`unwrapSingleton`). Object-in/object-out.
+  a single-child composition (`unwrapSingleton`). Object-in/object-out. (Surfaced as the
+  toolbar **Compose** menu — named to read distinctly from a data `transform`, below.)
+- `spec-data-transforms` — the data-pipeline counterpart: `transformSiteAt` resolves the view
+  the cursor is in and its `transform[]` range/count (for the CodeLens), `transformPlacementAt`
+  classifies a step slot (for the completion) — both tagged `shared` when the pipeline sits on a
+  composition parent, `view` on a unit. `DATA_TRANSFORMS` is the field-typed step catalog (filter,
+  calculate, aggregate, bin, timeUnit, window, joinaggregate, fold, lookup); each builder takes the
+  columns in scope and seeds each `${n:default}` tab stop with a type-appropriate one. A test
+  round-trips every snippet through `snippetToPlain` to assert it's valid JSON.
 - `spec-data` — the Vega-Lite data model: classify a `data` block (`classifyData`, mirroring
   `isNamedData`), the library reference name (`libraryRefName`), and the data binding in scope
   at a cursor path (`dataBindingAtPath`, honoring a view's data inheritance from its parent).
@@ -335,11 +343,11 @@ logic is pure `src/core/`; the Monaco glue is thin app-layer services.
   curated parameter signatures for the commonly-typed functions (what the registry can't supply).
 
 **Services (app, store-aware via `getState`):** `spec-transform-actions` (the
-wrap/simplify/add-view operations and their surfaces), `spec-dataset-hints` (data-column
-completion, hover, inlay providers), `spec-expression-hints` (expression completion,
-signature help, hover, and the diagnostic markers), `active-dataset` (`dataInfoAt(text,
-offset)` — the columns/types/stats plus derived fields the draft sees at the cursor).
-`SpecEditor` does the wiring.
+wrap/simplify/add-view operations and their surfaces), `spec-transform-scaffold` (the
+data-transform scaffold — a CodeLens plus a completion), `spec-dataset-hints` (data-column completion, hover,
+inlay providers), `spec-expression-hints` (expression completion, signature help, hover, and
+the diagnostic markers), `active-dataset` (`dataInfoAt(text, offset)` — the columns/types/stats
+plus derived fields the draft sees at the cursor). `SpecEditor` does the wiring.
 
 Decision rules:
 
@@ -407,7 +415,25 @@ Decision rules:
   `datum.` or `fn(` spans the whole `datum.`/`fn(` token; used as a completion item's range it both
   mis-targets the edit and filters every suggestion out (none start with `datum.`). A provider
   completing inside a string must build the replace range from the partial it parses itself — a rule
-  any future in-string completion (transform/param scaffolding) inherits.
+  the transform scaffold inherits (it parses the trailing element word off the line). This
+  `new Range(line, col − partialLen, line, col)` construction is now at three sites
+  (`spec-expression-hints`, `spec-dataset-hints`, `spec-transform-scaffold`); a shared
+  `replaceRange(position, partialLength)` helper is earned and should be extracted on the next touch.
+- **Data-transform scaffolding is a CodeLens (discoverable) plus a completion (accelerator), on
+  the one home the schema leaves bare.** A Vega-Lite data `transform` has three possible homes,
+  confirmed against the bundled schema (the `transform` array is on 16 spec types, i.e. every view
+  node): a `transform[]` **step**; the **inline** field props on an encoding channel
+  (`bin`/`timeUnit`/`aggregate`/`sort`); and a **new** pipeline on a bare view. We scaffold the step
+  home only, on the same "add only what the schema lacks" rule as `spec-dataset-hints`: the schema
+  already completes the inline channel keys and their enum values, and the `transform` key itself —
+  but never a ready, field-typed `{ "filter": … }`. The **CodeLens is the discoverable surface**
+  (a completion is invisible until provoked and competes silently with the schema's suggest items):
+  cursor-scoped like the composition CodeLens, it shows `＋ Add transform` on a view with no pipeline
+  and per-step `＋ filter`/`＋ aggregate`/… on the array, each clicking through Monaco's snippet
+  engine so the field-typed tab stops survive. The completion is the type-to-filter accelerator on
+  the same catalog. A step's `scope` (`shared` when the array is on a composition parent, so it
+  feeds every child) is surfaced so the placement is not a surprise, and comma affixing keeps the
+  array valid whether the slot is empty, between elements, or appended after one without a trailing comma.
 - **Code-action menu icons are kind-derived** (a wrench for the `refactor.*` kinds) — Monaco's
   `CodeAction` carries no icon field. Custom iconography lives only where it is supported:
   CodeLens titles (`$(codicon)`), completion-item kinds, and glyph-margin decorations.