fix(tui): prevent Tabby scroll jumps during streamed output

Problem

In Tabby, Kimi Code can jump the terminal viewport to the top while a response is streaming. The issue is most visible with long conversations and high-frequency output.

Reproduction

1. Open Kimi Code in Tabby.
2. Start or resume a conversation with enough transcript content to exceed the terminal height.
3. Send a prompt that produces a streamed response.
4. Observe that Tabby can leave the viewport at the top instead of following the active response.

Investigation

Kimi Code uses TuiMainScreen by default. Its full-redraw path emits CSI 2 J, CSI H, and CSI 3 J to clear and rebuild the main screen and scrollback. Streamed updates can exercise this path when changed content lies above the tracked viewport.

Tabby previously fixed a related fast-output viewport race in Eugeny/tabby#11102, released in v1.0.235. Using Kimi Code's existing alternate-screen renderer for Tabby avoids the destructive main-screen redraw and provides an application-owned scrollable transcript viewport.

Proposed fix

• Automatically select the existing TuiAltScreen renderer when TERM_PROGRAM=tabby.
• Preserve KIMI_CODE_TUI_FULL_SCREEN=0 as an explicit opt-out and =1 as an explicit opt-in.
• Add regression coverage for renderer selection and alternate-screen startup layout.
• Document the Tabby behavior in both environment-variable reference pages.

I have a tested patch ready and will open a PR after maintainer approval.

Start by tracing renderer selection between TuiMainScreen and TuiAltScreen, including how KIMI_CODE_TUI_FULL_SCREEN and TERM_PROGRAM=tabby are handled. Add regression coverage for renderer selection and alternate-screen startup layout, then update both environment-variable reference pages to document the Tabby behavior.