Preclude stale screen content in full-screen mode

[seh]

When ultraviolet runs in full-screen (alternate screen) mode with frequent updates to the same screen positions, relative cursor movements can interact in ways that cause the terminal to fail to overwrite old content, leaving stale screen content visible. Components like the bubbles library's spinner that update repeatedly on the same line can induce this problem.

The original cursor movement optimization logic assumed reasonably that using relative positioning operations (carriage return, cursor up, reverse index) would be safe and efficient for short distances when the cursor position is known. However, in full-screen mode with rapid updates, combinations of these relative movements can cause the terminal state to become inconsistent with what ultraviolet expects, resulting in content that should have been overwritten remaining visible.

Instead, force ultraviolet to always use absolute cursor positioning (e.g. CUP as ESC[<row>;<col>H) in full-screen mode. While the original relative movement approach works for simple or infrequent updates, absolute positioning is more resilient in full-screen mode because each movement is independent, rather than relying on accumulated state. The trade-off is slightly longer escape sequences, but this ensures reliable visual updates without stale content remaining visible.

Note that this changes only how full-screen mode works; the behavior of inline mode behavior remains unchanged.

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