refactor(reader): rebuild divina scroll pager

[everpcpc]

Problem

The DIVINA scroll reader had accumulated unstable behavior after the SwiftUI scroll-based pager was replaced. Page commits could fire mid-transition, first-load rendering could stall, rotation could lose the current page, tvOS input paths diverged between modes, and settings still exposed scroll effects that no longer existed.

Approach

Rebuild the scroll reader around native paged containers on iOS, macOS, and tvOS, with a shared paging session to separate rendered snapshots, committed reading position, and programmatic navigation. Move structural mode updates out of SwiftUI body evaluation, preserve the resolved current page across reader state rebuilds, and unify page presentation invalidation so visible cells refresh deterministically instead of relying on broad view reloads.

Scope

• Replace the old SwiftUI scroll pager with native ScrollPageView implementations for iOS/macOS
• Add shared paging/session/presentation coordination for commit timing and visible cell refresh
• Rework end-page/native page cell plumbing and share end-page presentation logic
• Fix rotation and startup regressions, including current-page preservation and stale index-path safety
• Simplify tvOS input routing and remove dead scroll-effect configuration that is no longer supported
• Document the repository preference for structural refactors over patch-style fixes in AGENTS.md

Validation

• make format
• make build