[naga] Add GLSL constant_id layout qualifier support and WGSL backend override output

[06wj]

Connections
No dependencies on other PRs. Please add related issues if any.

Description
This PR adds support for GLSL specialization constants (constant_id layout qualifier) in the Naga GLSL frontend and enables the WGSL backend to output override declarations.

1. GLSL Frontend

• Parse layout(constant_id = N) qualifier for const declarations
• Convert these to Naga Override instead of Constant
• Handle Override expressions in the constant evaluator

2. WGSL Backend

• Remove the "pipeline constants not supported" error
• Add write_override() method to output @id(N) override declarations
• Support Expression::Override in expression writers
• Implement write_override in TypeContext

3. Naming

• Add NameKey::Override variant for override naming

Testing

• Add spec-constant.frag test case with various specialization constant types

Squash or Rebase?

This branch contains a single functional commit and can be rebased directly.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.