Take a primitive's kind from its type, and keep images out of the rest

A `Primitive` now carries its own WGSL, and `PrimitiveRegistry` keys ids by
`TypeId`, so `Painter::primitive` takes only the value and the `RECT`,
`GLYPH` and `TEXTURE` constants are gone. Registering is what a first draw
does; the built-ins are seeded up front so first-draw order cannot decide
anything about them.

What a primitive samples is no longer something every registration states.
The glyph atlas and the one sampler moved into the shared group, which is
where a mask texture would go too, so a rect's pipeline has no texture in
its layout at all. Only a primitive whose type sets `TEXTURE` gets an image
group, and that is also what records the slot at write time -- so nothing
reads a `u32` back out of the instance payload.

Verified on the headless rig: the tabs example, two images added at runtime,
an image alone in a layer, and text spanning a four-layer atlas after the
array grew twice.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/widget/rect.rs

@@ -29,15 +29,12 @@ impl Rect {
 
 impl Widget for Rect {
     fn draw(&mut self, painter: &mut Painter) {
-        painter.primitive(
-            RECT,
-            RectPrimitive {
-                color: self.color,
-                radius: self.radius,
-                thickness: self.thickness,
-                inner_radius: self.inner_radius,
-            },
-        );
+        painter.primitive(RectPrimitive {
+            color: self.color,
+            radius: self.radius,
+            thickness: self.thickness,
+            inner_radius: self.inner_radius,
+        });
     }
 
     fn desired_width(&mut self, _: &mut SizeCtx) -> Len {

src/widget/text/edit.rs

@@ -73,7 +73,6 @@ impl Widget for TextEdit {
             let size = vec2(rect.width() as f32, rect.height() as f32);
             let top_left = vec2(rect.x0 as f32, rect.y0 as f32);
             painter.primitive_within(
-                RECT,
                 RectPrimitive::color(Color::SKY),
                 size.align(Align::TOP_LEFT).offset(top_left).within(&region),
             );
@@ -83,7 +82,6 @@ impl Widget for TextEdit {
         let size = vec2(caret.width() as f32, caret.height() as f32);
         let top_left = vec2(caret.x0 as f32, caret.y0 as f32);
         painter.primitive_within(
-            RECT,
             RectPrimitive::color(Color::WHITE),
             size.align(Align::TOP_LEFT).offset(top_left).within(&region),
         );