Grid Multi-Body: Fix Null Data Store and Consolidate Loading Mask

Problem

When locked columns are configured (like the recent DevIndex test), GridContainer instantiates bodyStart and bodyEnd but fails to explicitly pass the store reference downward. This causes a fatal TypeError: Cannot read properties of null inside GridBody.createViewData() when it attempts to evaluate .isLoading.

Additionally, loading masks historically target the inner GridBody. In a Multi-Body layout, this would create fragmented loading spinners across each discrete column body. The mask must be promoted to target the unified bodyWrapper.

Implementation Plan

• GridContainer: Update bodyStart and bodyEnd instantiation loops to explicitly map store and useInternalId.
• Loading Mask Promotion: Re-route isLoading behavior so that it masks the bodyWrapper instead of fragmenting across all three inner bodies.