[P1] Add golden-path integration tests for import, hold, window, save, and restore

[nkanf-dev]

Summary

Create store-level integration coverage for the main reading lifecycle so state synchronization regressions are caught early.

Why

The most failure-prone behavior in this app is not a single component; it is the coordination between stores during import, page holding, windowing, and restoration.

Current evidence

• Save/restore and linked window behavior span bookStore, heldStore, windowStore, and workspaceStore.
• Existing tests are either outdated or blocked by environment issues.

Scope

• Add integration tests for: import document, hold page, open floating/split windows, save snapshot, reset stores, restore snapshot.
• Assert linked window IDs, active window, current page, and scale synchronization.
• Reuse realistic but lightweight test fixtures.

Acceptance criteria

• The golden path is covered by automated tests.
• Cross-store synchronization regressions fail tests deterministically.
• Tests are readable enough to serve as behavior documentation.

Out of scope

• Full visual regression testing.