v0.1.6: emoji spacer overflow retriggered SIGWINCH replay

Bug fix: a wide character (emoji) in a full-width TUI box made GhostShell send a 1-row SIGWINCH in a loop. The app then replayed the whole conversation until the prompt came back.

libghostty-vt already knew the glyph was 2 cells (ghostty_unicode_grapheme_width). The Sublime line also kept Ghostty's spacer space, so the box was one em too wide, the H-scrollbar stole viewport height, and measured rows flipped 47↔48. The spacer is dropped now. A sub-cell jog can remain if the font's emoji advance is not exactly 2 × em_width; that is not enough to pop the scrollbar.

Also: scrollback_history_size is still lines; it is converted to bytes at terminal_new because libghostty-vt's max_scrollback is a byte budget (ghostty#12769). The session text log is the painted tab without the 300-line cap.