[KISS/Clean Code] Extract grid column, change-mapping and enabled-cell rendering policies

[KeyffMS]

Audit finding

ApplicationAssignmentsGrid is a useful presentation boundary, but one class still owns:

• DataGridView and column construction;
• selector option population;
• row writing and lookup;
• column-name-to-command translation;
• recoverable data-error classification and diagnostics;
• custom enabled-cell painting;
• selection and empty-state publication.

The result is a large control whose data contract, rendering rules and event decoding change for unrelated reasons.

Target state

Keep ApplicationAssignmentsGrid as the UserControl boundary while extracting only stable focused collaborators, for example:

• column/selector configuration;
• typed cell-change mapping;
• enabled-state cell renderer;
• selector data-error policy.

Avoid introducing a general UI framework.

Acceptance criteria

• Column construction and selector option definitions are outside the main control lifecycle code.
• Mapping a changed cell to ApplicationAssignmentChange is independently testable without raising WinForms events.
• Enabled-cell painting is isolated and testable through its input state/geometry policy.
• Recoverable selector-error classification remains explicit and continues to emit structured diagnostics.
• ApplicationAssignmentsGrid remains the owner of selection, binding, row updates and typed change publication.
• Existing keyboard, focus, selection, rollback and empty-state behavior remains unchanged.
• No reflection-based or global DataGridView workaround is introduced.