fix: Stop overloading `d.location` for vertex `shaderLocation`

fix: Stop overloading d.location for vertex shaderLocation

In WGSL, decorators like @size or @align can only decorate struct members.

Issue #2754 pointed out that we could pass decorators to arrayOf(), which usually had no effect and was misleading.

PR #2781 partially fixed this by rejecting non-@location decorations on array element types. @location remains allowed as an exception, because it is used with vertexLayout.

Problem

d.location currently means two different things depending on context:

• On a struct / IO member, it maps to the WGSL @location attribute.
• In a vertexLayout, decorating an array element with d.location sets the pipeline’s shaderLocation attribute.

Moreover, due to this exception, d.location can always be passed to arrayOf(), which might be misleading.

Proposal

Make decorators single-purpose and find another way to pass shaderLocation to the vertexLayout, so arrayOf() no longer needs a special case for @location.

Start by tracing how d.location flows through arrayOf() and vertexLayout, comparing struct or IO member handling with vertex shaderLocation handling. The work is done when decorators are single-purpose, vertexLayout still receives shaderLocation, and arrayOf() no longer needs a special case for @location.