EnterAltScreen doesn't create true alternate screen in Terminal.app - adds blank spaces instead

**Describe the bug**
When using `tea.EnterAltScreen` in Terminal.app on macOS, the alternate screen is not working correctly. Instead of creating a true alternate screen buffer (like other TUI applications do), BubbleTea appears to simulate it by adding blank spaces at the top of the terminal, pushing existing content up. The user can still scroll up to see the previous terminal content.

**Setup**
- OS: macOS
- Shell: zsh
- Terminal Emulator: Terminal.app (default macOS terminal)

**To Reproduce**
Execute in the bubbletea repo, `examples/fullscreen/main.go`

**Expected behavior**
- Previous terminal content should be completely removed (not scrollable)

Start by running examples/fullscreen/main.go in macOS Terminal.app with zsh and compare its behavior with other TUI applications. Trace how tea.EnterAltScreen is handled, then verify that the previous terminal content is no longer visible or scrollable when the alternate screen is entered.

Written by the indexing model from the issue text.