Expose stable Bazel visibility target for generated scan

[brentleyjones]

Summary

Add a stable @periphery//bazel:generated package group that includes a generated-repo package group created at repository rule evaluation time. This gives Bazel projects a stable visibility label they can reference without depending on the generated scan package layout.

This is useful for --bazel-check-visibility, which disables Periphery's default --check_visibility=false behavior. With a stable wrapper target available, projects can grant visibility to Periphery's generated scan target explicitly and use checked visibility safely without relying on the ephemeral @periphery_generated root package.

Changes

• add @periphery//bazel:generated as a stable package group wrapper
• make @periphery_generated expose //visibility:package_group at repository-rule creation time
• update the Bazel docs to point --bazel-check-visibility users at the stable wrapper target
• add a changelog entry for the new visibility target

[brentleyjones]

@ileitch