fix(tui): stale content after tree navigation

[Perlence]

Problem

When navigating the session tree with "No summary", stale messages from the old branch remain on screen and duplicate on repeated navigation.

Here's a repro case that the clanker has crafted:

repro-stale-tree-nav.zip

And here's the recording of the bug in that session and the fix:

Screen.Recording.2026-03-14.at.17.25.57.mov

Root cause

The differential renderer decides whether to fall back to fullRender by checking if firstChanged is above the viewport:

const previousContentViewportTop = Math.max(0, this.previousLines.length - height);
if (firstChanged < previousContentViewportTop) {
    fullRender(true);

This uses previousLines.length to estimate the viewport position. But maxLinesRendered, which determines the actual viewport, can be larger than previousLines.length when transient UI components (tree selector, extension selector) inflated it during the same interaction. The tree selector adds ~40 extra lines, then closes, but maxLinesRendered doesn't shrink.

This was done in 0925faf which changed the check from firstChanged < viewportTop (using maxLinesRendered) to firstChanged < previousContentViewportTop (using previousLines.length) to avoid false positive redraws when appending lines after content shrunk. However, the differential renderer genuinely can't address lines above maxLinesRendered - height.

Fix

Use prevViewportTop (derived from maxLinesRendered) for the check, restoring the original correct behavior:

if (firstChanged < prevViewportTop) {
    fullRender(true);

This may trigger a few more fullRender calls when maxLinesRendered is inflated, but those redraws are necessary for correctness.