[KeyffMS]

Addresses #119.

Changes:

• moves DataGridView column construction and selector-option definitions into ApplicationAssignmentsGridColumns;
• moves cell-value to typed ApplicationAssignmentChange decoding into independently testable ApplicationAssignmentCellChangeMapper;
• moves enabled-cell indicator geometry/painting into ApplicationAssignmentEnabledCellRenderer;
• moves selector data-error classification and diagnostics into ApplicationAssignmentSelectorErrorPolicy;
• keeps ApplicationAssignmentsGrid responsible for selection, binding, rows, rollback updates, empty state and typed change publication;
• preserves existing keyboard/editing behavior and structured diagnostics;
• adds focused policy tests and keeps configuration-grid regression tests;
• introduces no global DataGridView workaround or new UI framework.