fix(render): adapt mesh + hatch load colours to background

`adapt_to_bg` (pure black → white on dark bg, pure white → black on
light bg) was only running through `render_style`, which the wire and
per-frame hatch paths already use. The mesh population paths and the
file-load hatch path went straight through `aci_to_rgba`, so:

- ACIS solids / regions / bodies appeared in their raw ACI colour
  even when that colour was pure white on a white paper bg.
- The initial hatch upload used the unadapted colour for the first
  frame after load (`synced_hatch_models` then re-ran adaptation per
  frame, so the issue cleared after one redraw — but visible).

Now every mesh / hatch creation site funnels through
`render_style_for` + `adapt_to_bg`:

- `build_derived_caches` (load): uses the default model bg as the
  adaptation target.
- `populate_meshes_from_document`: uses `self.render_style` so the
  per-frame paper / model bg selection applies.
- New `Scene::recolor_meshes` rewrites every cached mesh's colour
  field through `render_style`; called from `BACKGROUND` (after the
  user changes bg) and `set_current_layout` (Model ↔ paper bg
  switch) so ACIS mesh colour tracks the live bg without re-
  tessellating ACIS geometry.

Wipeout fills stay bg-coloured by design (that's their job). Raster
images skip adaptation — pixel data is not entity colour.

src/app/commands.rs

@@ -54,7 +54,12 @@ impl OpenCADStudio {
                     // Wire colour adaptation (`adapt_to_bg`) reads the bg
                     // at tessellation time, so the cached wires need to
                     // refresh — otherwise a light→dark bg flip leaves
-                    // black lines invisible against the new bg.
+                    // black lines invisible against the new bg. Meshes
+                    // bake colour into per-vertex GPU buffers at upload
+                    // time; `recolor_meshes` rewrites the CPU side so
+                    // the next epoch-driven re-upload picks up the new
+                    // colour.
+                    self.tabs[i].scene.recolor_meshes();
                     self.tabs[i].scene.bump_geometry();
                     self.command_line
                         .push_output("Background reset to default.");
@@ -69,6 +74,7 @@ impl OpenCADStudio {
                         self.tabs[i].bg_color = Some([r, g, b, 1.0]);
                         self.tabs[i].scene.bg_color = [r, g, b, 1.0];
                     }
+                    self.tabs[i].scene.recolor_meshes();
                     self.tabs[i].scene.bump_geometry();
                     self.command_line.push_output(&format!(
                         "Background: rgb({}, {}, {})",