Fix renderer position preservation regressions

[emil-e]

Summary

• preserve saved markers/window positions correctly across batched changed-length renderer spans
• keep logical cursor positions updated even when the terminal cursor is hidden
• avoid snapping focus clicks to point-max when no cursor position is available

Verification

• make .build/tests/native-ghostel-render-test.ok
• make .build/tests/native-ghostel-mouse-paste-test.ok