Refactor: isolate Studio page-layout settings regression module

[rhamenator]

Parent: #2564

Scope

Extract one cohesive contiguous page-layout settings regression family from tests/test_studio_host_json_settings_page_layout_fields.cpp into a focused included test module while preserving the existing aggregate test executable and call order.

Acceptance

• The selected page-layout settings family has one clearly named focused module.
• The aggregate test target retains its existing invocation and behavior.
• Focused native test evidence is retained.
• No Studio, host JSON, report/label, localization, or machine-contract behavior changes.

Traceability

Structural-only test-ownership improvement under the oversized-module umbrella; no new UI or compatibility claim.

[rhamenator]

Completed by #5231 (merged 2026-08-25). The structural-only GRIDV page-layout regression-module extraction retained its focused 2/2 CTest evidence and green protected matrix; no product behavior or machine-contract change.