Add `woodshop render <file.png>` for headless viewing

Saves an off-screen PNG of the scene (labels, grid, isometric) so the model can
be inspected without an interactive GUI window — useful over SSH or when
woodshop-view can't open a display. 44 tests passing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/woodshop/cli.py

@@ -146,6 +146,14 @@ def cmd_cutlist(scene: Scene, args) -> str:
     return format_cutlist(scene)
 
 
+def cmd_render(scene: Scene, args) -> str:
+    from .viewer import render_to_file  # lazy: pulls in pyvista
+    if not scene.parts:
+        return "Nothing to render — the scene is empty."
+    path = render_to_file(scene, args.path)
+    return f"Rendered {len(scene.parts)} part(s) to {path}."
+
+
 def _describe_part(p) -> str:
     bits = [f"{_fmt_len(p.length_in)} {p.stock}"]
     if p.name:
@@ -257,6 +265,10 @@ def build_parser() -> argparse.ArgumentParser:
     sp.add_argument("path", help="Output file, e.g. table.stl or table.step")
     sp.set_defaults(func=cmd_export)
 
+    sp = sub.add_parser("render", help="Save a PNG image of the scene (works headless)")
+    sp.add_argument("path", help="Output image, e.g. table.png")
+    sp.set_defaults(func=cmd_render)
+
     sub.add_parser("cutlist", help="Show the cut list / bill of materials").set_defaults(func=cmd_cutlist)
     sub.add_parser("undo", help="Undo the last operation").set_defaults(func=cmd_undo)
     sub.add_parser("clear", help="Clear the scene").set_defaults(func=cmd_clear)
@@ -272,7 +284,7 @@ def main(argv: list[str] | None = None) -> int:
     except (SceneError, ValueError, KeyError) as exc:
         print(str(exc).strip('"'), file=sys.stderr)
         return 1
-    if args.command not in ("status", "export", "cutlist", "save", "projects"):
+    if args.command not in ("status", "export", "cutlist", "render", "save", "projects"):
         scene.save(args.scene)
     print(message)
     return 0

src/woodshop/viewer.py

@@ -77,6 +77,22 @@ def _render(plotter, scene: Scene) -> None:
     plotter.add_axes()
 
 
+def render_to_file(scene: Scene, path, window_size=(1100, 800)) -> str:
+    """Render the scene to a PNG (off-screen) — works headless / over SSH."""
+    import pyvista as pv
+
+    _quiet_vtk()
+    pv.OFF_SCREEN = True
+    plotter = pv.Plotter(off_screen=True, window_size=window_size)
+    plotter.set_background("#2b2b2b")
+    plotter.enable_parallel_projection()
+    _render(plotter, scene)
+    plotter.view_isometric()
+    plotter.screenshot(str(path))
+    plotter.close()
+    return str(path)
+
+
 def run(scene_path: Path | None = None, poll_s: float = 0.3) -> None:
     import pyvista as pv