fix: preserve current page across reader state regeneration

[everpcpc]

Problem

Reader state regeneration could overwrite the actual current page with the primary page ID of the resolved view item. When the current item was a dual-page item, that collapsed the position back to the first page of the pair and could cause restore/navigation to drift away from the page the user was actually on.

Approach

Preserve the preferred current page whenever it still belongs to the resolved view item, and only fall back to the item's primary page ID when needed.

Scope

• add a helper to resolve the current page ID while preserving the preferred page when it is still part of the current view item
• use that helper during reader state regeneration
• use that helper when committing the current position from a resolved view item

Validation

• make format
• make build-ios