Function resolveRosterUnit

Resolve a roster's unit entry against the dataset, returning the linked UnitView. Returns undefined when:

• the roster's ref.id is null (the importer couldn't match the unit), or
• the id doesn't appear in the dataset (e.g. the roster was authored against an older dataslate than the bundled one).

Doesn't surface diagnostics — the caller already has them on the roster's own diagnostics field.