Report a failed spawn, drop the paste wrapper and the unbound close command, sync docs

- vterm_open shows an error dialog when the command cannot start,
  instead of a tab that silently vanishes
- on_text_command hands paste straight to vterm_paste_text;
  VtermPasteCommand only forwarded
- VtermCloseCommand was bound nowhere; closing the tab already kills
  the process
- wheel_lines fallback matches the shipped 1; README said three
- Architecture.md: per-theme scheme files and managers, command table

README.md

@@ -87,8 +87,8 @@ fullscreen UI before it enables mouse reporting).
 Sublime raises no wheel event at all for the trackpad (sublime_text#1838): set
 a utility such as [LinearMouse](https://linearmouse.app/) to discrete scrolling
 for Sublime Text and it arrives as wheel notches like any other. A notch sends the
-app three scroll lines, as iTerm2 does; `wheel_lines` lowers that when a swipe
-scrolls too far. While an app owns the
+app one scroll line; `wheel_lines` raises that (iTerm2 sends 3) when the wheel
+feels slow. While an app owns the
 alternate screen the scrollback above it is folded away, kitty-style, and
 unfolds when the app leaves it.