Retain a stable message window in channel timelines

[tlongwell-block]

Summary

• replace continuous unknown-height Virtua virtualization with a Zulip-style retained window of 250 real message rows
• shift the retained window only when the first visible semantic message approaches a 50-row edge
• preserve that message ID and its viewport-relative top exactly once across each discrete window transaction
• realize search, unread, and deep-link targets by moving the retained window around the target before centering it
• keep appends inert while reading history, preserve history-prepend anchoring, and remove the Virtua dependency

Validation

• pnpm --dir desktop typecheck
• pnpm --dir desktop build
• desktop unit suite: 2,274 passed
• pnpm --dir desktop exec playwright test tests/e2e/virtualization.spec.ts --project=smoke: 7 passed
• Biome clean
• push hook: desktop, mobile, and Tauri suites passed; aggregate hook remains blocked by the unrelated Rust 1.89 vs sqlx 1.94 requirement
• pre-commit remains blocked by the existing unrelated readiness.rs 1549-vs-1546 line limit

Known bound

The late-measurement watcher remains active for at most 3 seconds after a prepend. Without a component-local native-input classifier, a user who deliberately scrolls less than two viewports and then settles during that window can be restored toward the prepend anchor. We intentionally do not infer intent from scroll events because Virtua programmatic compensation emits the same events and would disable needed correction.