fix(naga): zero initialize loop-local var declarations in each iteration

[ruihe774]

Connections
Fixes #9489.

Description
Loop-local var declarations without explicit initializers are zero-initialized in each iteration.

Testing
A new naga validation test is added to ensure ZeroValue is emitted in each iteration for such variables.

Squash or Rebase?
Only one single commit

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.