fix(chat): eliminate conversation switch flicker

[AprilNEA]

What changed

• render the newly selected conversation from the live selection instead of deferring the outgoing conversation
• keep virtua's external-scroll offset synchronized while row measurements settle before the first paint
• extend the Chromium smoke flow to assert that the first rendered update already shows the selected title, the latest turn, and a bottom gap within 2 px

Root cause

The session surface still derived its active conversation from useDeferredValue(selectedId). That deferral was introduced with the old View Transition path, but remained after View Transitions were removed, so a click intentionally committed one more frame of the outgoing conversation.

After that stale frame was removed, the existing synchronous DOM snap exposed a second timing gap: virtua tracks an external scroller from scroll events. As measured row heights settled, the DOM reached the bottom before the virtualizer learned the new offset, leaving the first paint about 9.75 px above the tail.

Impact

Switching conversations now paints the selected conversation directly at its latest message. The optional smooth-follow setting from #385 remains unchanged for users who explicitly enable it.

Verification

• pnpm check:ci
• pnpm test — 302 files passed, 3 skipped; 2,435 tests passed, 5 skipped
• pnpm -F @linkcode/webview e2e:browser
• mutation check: restoring useDeferredValue fails with First paint still showed "Wire the workbench to the daemon"
• timing check: omitting the virtualizer scroll notification fails with a 9.75 px first-paint bottom gap

Tracks CODE-259.

[linear-code]

CODE-259