feat(plugins): investigate DeepSeek conversation reading position restore

Proposal

Investigate restoring a reading anchor scoped to the same tab and conversation. Require stable message identity and virtualization-aware materialization; restoring raw scrollTop alone is insufficient. Avoid persisting transcript text or introducing cross-device sync. Cancel restoration on manual scroll and route changes. Confirm account isolation and source contract before implementing.

Contribution workflow

The contributor requests implementation where feasible. This issue records scope before code. Maintainer confirmation of new primitive contracts is still needed; no maintainer approval is implied by opening this issue. New PRs will be submitted only after applicable checks pass, with real-browser gaps resolved rather than labelled as complete. Related DeepSeek integration: #960 and #995.