Draw each primitive with its own pipeline, registered rather than declared

The `primitives!` macro, `PrimitiveData`, `PrimitiveVec`,
`PrimitiveBuffers`, the `Primitive` trait and the shader's dispatch
switch are gone. A primitive is now a registration: its WGSL and,
implicitly, the size of the entry that WGSL reads. Everything else --
its instance list, its free list, its buffers, its bind group and its
pipeline -- follows from that, so adding one is a `register` call and a
shader file, with nothing per-type to remember and no cross-type
dispatch to extend.

Nothing dispatches dynamically. Push, free, renumber, upload and draw
are identical for every primitive; what differs is the entry size and
the pipeline, which are data. So `InstanceList` carries a runtime
stride and its instances' data as bytes, and one concrete type serves
every primitive and the textures. Measured against a typed list it
costs 0.2ns per write, where a trait object costs 1.6ns.

Because each type has its own list, an instance's index is also its
data index: `@builtin(instance_index)` replaces the `idx` field, the
`binding` field goes with the switch, and `PrimitiveInstance` drops from
28 bytes to 20. `PrimitiveVec`'s free list merges into the instance
list's, so an instance and its data are freed by one `swap_remove`
rather than two arenas kept in step.

`shader.wgsl` becomes `shader/prelude.wgsl` plus one file per primitive.
The prelude carries the window, masks, sampled texture, vertex shader
and `masked()`, and is compiled ahead of each primitive's own source --
which is also what a caller's own primitive would be. Masks move back
into group 0 beside the window uniform, since every pipeline shares one
layout.

Within a layer, types now draw in registration order: a rect under a
glyph under a texture. Order within a layer was never meaningful --
freeing an instance swaps another into its place -- so this replaces an
accident with a defined order, and backgrounds land under their content.

Verified with a headless run per case: text over its own rect and an
image over its own rect in one layer, a masked stack clipping, the
text-layout tab, and adding three images and deleting one.

core/src/ui/render_state.rs

@@ -245,7 +245,7 @@ impl UiRenderState {
     pub fn debug_layers(&self) {
         for ((idx, depth), draws) in self.layers.iter_depth() {
             let indent = " ".repeat(depth * 2);
-            let primitives = draws.primitives.instances().len();
+            let primitives: usize = draws.primitives().iter().map(|l| l.instances().len()).sum();
             let textures = draws.textures.instances().len();
             println!("{indent}{idx}: {primitives} primitives, {textures} textures");
         }