fix(glsl-out): add parens. to atomicSub negation

[ErichDonGubler]

Connections

• Resolves [naga] GLSL backend emits invalid -- token when atomicSub operand starts with a negation #9883.

Testing

New snapshot test added. Most of this PR is snapshot testing.

Squash or Rebase?

Squash.

Checklist

• I self-reviewed and fully understand this PR.
• WebGPU implementations built with wgpu may be affected behaviorally.
• Validation and feature gates are in place to confine behavioral changes.
• Tests demonstrate the validation and altered logic works.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and doesn't make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.

[ErichDonGubler]

@zero0zero7, does this fix work for you?