Extends ConfigDisplayConfig and UiState so the four new display flags
flow from `config.get {key:"full"}` into the nanostore. applyDisplay is
exported to keep the fan-out testable without an Ink harness.
Defaults mirror v1 parity: streaming + inline_diffs default true
(opt-out via `=== false`), show_cost + show_reasoning default false
(opt-in via plain truthy check).
