Apple boardingPass renders a transit-direction arrow next to the primary
field; with a single field the arrow points into empty space and visually
crowds the value. To support the natural origin→destination layout (or any
two-field pairing), `primary()` now returns `Vec<Field>` instead of a
single `Field`. Existing callers wrap their value in `vec![…]`.

- `WalletSubject::primary` signature widened to `Vec<Field>`
- Apple manifest emits all returned entries into `primaryFields`
- Google `eventTicketObject` consumes only the first entry (single-headline
  slot), defaults to empty string when no primary fields are supplied
- Test fixtures updated

BREAKING: implementors must wrap their primary field in `vec![…]`.