refactor: thread card-state struct through the item-data round-trip

Second step of #147. get-item-data now returns an org-drill-card-state and store-item-data takes one, so the six recall fields move as named slots instead of a positional list. The three call sites (smart-reschedule, hypothetical-next-review-date, copy-scheduling-to-marker) read scheduler inputs through accessors and build a struct for the store, which removes the hand re-ordering between the get-shape and the store-shape.

Behavior is unchanged. The legacy LEARN_DATA read path and the virgin-item sentinel are preserved field-for-field, and store takes just the struct because its last-interval slot already holds the interval to persist. The schedulers still take positional args; they adopt the struct in the following commits.

I updated the round-trip, integration, and setup-helper tests to build and read the struct via a small list-view helper, so the existing expected-value assertions stay readable.

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