Preserve reading position while loading unread messages

[witbrock]

Merge decision: ready — loading preserves the reader's visible message, and an explicit unread jump moves only once its target is loaded.

User outcome

Fixes native task #V#task_agent_59405b42688c3530eda8da4e5e057bb8. Default and background message loading no longer scroll to the bottom or implicitly navigate to unread. During a requested unread jump, intermediate pages preserve the visible contribution until the target is available.

Material changes

• Capture the visible contribution and its offset immediately before rendering, respecting scrolling during network requests.
• Restore that position synchronously before asynchronous cartouche hydration; preserve existing content while a reload is pending.
• Retain explicit unread and latest-message navigation and existing conversation-switch cancellation.
• Extend unit and browser regression coverage for default, background and delayed paginated loads.

Evidence

• 45 tests passed across message exchange, single-thread opening, reply context and send-failure suites; three additional cartouche/recommendation tests passed.
• Playwright using candidate production modules, chat template and CSS with synthetic HTTP responses passed at 390px and 1280px. Default top and near-bottom background position preserved; intermediate page retained the anchor within one CSS pixel; target focused and visible only after availability.
• Static frontend lint and git diff --check passed.
• Adjacent messageStreamNavigation suite: 3 copy-reference tests fail identically on baseline 93fc8967bf3b1a9548f0ddadd7ed2aae8887c68e; its task-button test passes. No copy-reference changes in this PR.
• Retained browser measurements/screenshots: .run/unread-navigation/ in the assigned worker checkout. This is synthetic browser acceptance, not live authenticated acceptance.
• Inspected source image #V#computer_file_copy_5debdaa8512445268f771f0af0dcac07, SHA-256 07dfd5e3e471af1c81bf506760e535c8b7161f29e46f5d7af21eecfa13060568.

Ship boundary

Tier 1, deterministic frontend behaviour. Ship criteria are preservation during loading and deferred explicit navigation, without regressions in affected rendering paths. Authentication, read-state semantics, and conversation-reference copying are outside scope. No deployment requested by this task.