Headless pictures draw on the GPU, with the viewport's pipelines (CPU rasterizer as last tier) (#1555)

Thanks @jyviko.

docs/mcp.md

@@ -18,7 +18,7 @@ document remains the reference implementation today.
 ## Start it
 
 ```sh
-# Headless: an in-process design session, no window (CPU renders for screenshots).
+# Headless: an in-process design session, no window (screenshots are drawn on the GPU).
 solidcraft-cli mcp
 
 # Headless, starting from a design file or a command script.
@@ -149,7 +149,7 @@ They cover the workflow, the command shapes, how to verify with `measure`, and t
 | `measure` | `body?` or `items?` | Volume (mm³), area (mm²), centre of mass, bbox and topology counts per body, with totals. With `items` (one or two selections as `execute` takes them: `face`, `edge`, `vertex`, `sketch_curve`, `profile`), the picked geometry instead: see below. |
 | `body_topology` | `body` | Edges (index, midpoint, length, ends) and faces (index, name, area, centroid, normal, `point`, and for a cylindrical face its `cylinder`: `origin`, `axis`, `radius`). An edge midpoint is the `[x,y,z]` that picks that edge for fillet and chamfer. A face's `point` lies on it and picks it (its centroid may not: that of a face with a hole is in the hole); a face's `name` picks it for the commands that take faces (`face.delete`, `solid.shell`, `solid.draft`, `solid.offset_face`, `solid.split_face`, `solid.replace_face`, and `face` of `sheet.convert`). |
 | `set_parameter` | `name`, `value` (number or expression), `unit?`, `comment?` | Create or change a user parameter and recompute; reports `recomputed` (features rebuilt), `restored` (features whose earlier result for the same inputs was reused, e.g. a value set back) and features that now fail. |
-| `screenshot` | `view?` (iso, top, front, back, bottom, left, right, home, fit, or a named view saved with `view.save`), `camera?` (`{target, yaw, pitch, distance, fov?}`, as `view.apply`), `path?`, `width?`, `height?`, `source?` (window, model), `grid?`, `axes?` | A PNG image content block. Headless: CPU render fitted to the model, or from the named view or camera. Bridged: the app window (default) or a model render with the app's camera; a named view or camera moves the app's viewport there first (`execute` `view.apply` does the same without a screenshot). `grid: false` and `axes: false` leave the ground grid (it lies on Z 0 and cuts through a part that straddles it) and the origin's axes out of a model render; axes hidden with `browser.visibility` stay out whatever `axes` says. Bridged, a screenshot that gives `grid` or `axes` is a model render, which follows the app's Grid and Origin switches for what it does not give; `source: "window"` with either is an error. |
+| `screenshot` | `view?` (iso, top, front, back, bottom, left, right, home, fit, or a named view saved with `view.save`), `camera?` (`{target, yaw, pitch, distance, fov?}`, as `view.apply`), `path?`, `width?`, `height?`, `source?` (window, model), `grid?`, `axes?` | A PNG image content block. Headless: a render with the viewport's pipelines on a headless GPU device (see `solidcraft-cli gpu`; with no adapter, the CPU rasterizer, without analysis colours), fitted to the model, or from the named view or camera; the reply's `renderer` says which (`gpu`, `gpu-software`, `cpu`, with a `warning` for `cpu`). Bridged: the app window (default) or a model render with the app's camera; a named view or camera moves the app's viewport there first (`execute` `view.apply` does the same without a screenshot). `grid: false` and `axes: false` leave the ground grid (it lies on Z 0 and cuts through a part that straddles it) and the origin's axes out of a model render; axes hidden with `browser.visibility` stay out whatever `axes` says. Bridged, a screenshot that gives `grid` or `axes` is a model render, which follows the app's Grid and Origin switches for what it does not give; `source: "window"` with either is an error. |
 | `export` | `path`, `format?` (step, 3mf, 3mf-tagged, stl, stla, obj, urdf), `bodies?`, `mesh_prefix?`, `effort?`, `velocity?` | STEP, 3MF or mesh export; URDF writes the whole assembly with its meshes next to it (`docs/urdf.md`). |
 | `undo`, `redo` | — | Undo / redo one change. |
 | `crash_reports` | `id?`, `limit?` | The crash reports on this computer, newest first; with `id`, the whole report (backtrace, command, recent commands, `design_path`/`replay_path`). See [crash-reports.md](crash-reports.md). |