vxfw: add opt-in primary-screen history mode (preserve terminal scrollback)

[Fristender]

Summary

This PR adds an opt-in mode for vxfw.App that runs in the primary screen and preserves terminal scrollback/history, while keeping default libvaxis behavior unchanged.

What changed

• Added vxfw.App.Options.primary_screen_history: bool = false

  • Default remains existing behavior (alternate screen + existing interaction model).
  • When true, app runs in primary screen and preserves history.

• Wired vxfw.App.run to:

  • Use alt screen only when primary_screen_history == false.
  • Set vx.opts.preserve_screen_on_exit based on the new option.
  • Ensure an initial winsize is applied before first layout/render.
  • Reserve terminal space in primary-screen mode so prior shell output is retained above the app.
  • Trigger an initial redraw on startup.

• Updated Vaxis behavior to support preservation:

  • Added Vaxis.Options.preserve_screen_on_exit (default false).
  • In resetState, when preservation is enabled on primary screen, avoid clearing the frame and move to a new line.
  • In resize, avoid erasing below cursor in preserved primary-screen mode.

Behavior

• Default (.primary_screen_history = false): unchanged from current libvaxis defaults.
• Opt-in (.primary_screen_history = true):
  • Previous terminal output remains in scroll history.
  • Final TUI frame remains visible in history after exit.

Notes

Mouse capture/input policy changes are intentionally not included in this PR.