fix(tui): preserve scrollback on content-driven full redraws

[mjapple8]

Problem

In normal-screen mode, pi-tui renders conversation history into the terminal's scrollback buffer so users can scroll up to read earlier output. When any rendered line above the previous viewport changes (e.g. streaming markdown re-flowing while the message grows past the visible area), TuiMainScreen.doRender() falls back to fullRender(true), which unconditionally emitted \x1b[2J\x1b[H\x1b[3J — and \x1b[3J wipes the entire scrollback.

Result: while viewing a session, the view "jumps to the top" and scrollback (conversation history, shell history) disappears; during streaming the redraw fires repeatedly so the view keeps jumping.

This is the same class of bug reported in #7304, #4626 and #5576. An earlier fix (PR #4204) targeted the old TUI class in packages/tui/src/tui.ts; it was lost during the refactor that split the renderer into tui-main-screen.ts / tui-alt-screen.ts.

Fix

Split the clear flag of fullRender into three modes:

mode	behavior	used for
false	first render; writes everything	initial frame
true	clear viewport (\x1b[2J\x1b[H) but PRESERVE scrollback; write only the last height lines so earlier content isn't pushed into scrollback a second time	content-driven redraws: firstChanged < prevViewportTop, height change, clear-on-shrink
"scrollback"	clear viewport and scrollback (\x1b[2J\x1b[H\x1b[3J); write everything	width changes only (wrapping changes make prior scrollback visually wrong)

maxLinesRendered is now reset only on a scrollback wipe; otherwise it keeps tracking growth since scrollback still holds the content.

Tests

Added packages/tui/test/tui-scrollback-preserve.test.ts with three cases (verified locally against a real xterm.js headless terminal):

1. Preserves shell scrollback across a content-driven full redraw — pre-TUI shell history survives when a line above the viewport changes (fails on the old code: all history wiped).
2. Does not duplicate content into scrollback on a viewport-only redraw.
3. Clears scrollback on width change — stale wrapped scrollback is still removed when wrapping is invalidated.

[github-actions]

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