Implement validation pipeline step using VEntry/VFS

[DamianReeves]

Background:

• Validation exists in pkg/tooling/validation but is not a pipeline step.

Scope:

• Create a pipeline step that locates IR from VEntry/VFS inputs.
• Validate via pkg/tooling/validation, emitting diagnostics and artifacts.

Acceptance criteria:

• Pipeline step returns Validation Result as an artifact.
• Diagnostics include validation errors with step attribution.