[DRY/Clean Code] Introduce an application-assignment row model and unified grid change contract

[github-actions]

Audit finding

ApplicationProfilesGrid repeats executable-path validation in four event-args classes and repeats domain-to-cell mapping in AddRow and UpdateApplication.

Selector columns repeat construction properties, and change dispatch is a long column-name conditional that must be edited for every field.

The grid therefore owns too many representations of the same row truth.

Target state

Introduce:

• an immutable ApplicationAssignmentRow view model;
• one mapper from assignment/settings to row;
• one typed ApplicationAssignmentChange contract (or focused equivalent);
• one selector-column factory/definition table.

The grid must remain independent of persistence and transaction logic.

Acceptance criteria

• Row creation and row update use the same mapper.
• Executable-path validation is defined once.
• Adding a new editable assignment field requires one column definition and one typed mapping path.
• No object-typed change payload is introduced.
• Data-error recovery remains restricted to selector formatting/display contexts.
• Existing selection preservation and failed-cell restoration behavior remains covered by tests.