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

tests/test_render.py

@@ -455,6 +455,8 @@ Terminal._terminals[id(terminal)] = terminal
 terminal.view.id = lambda: id(terminal)
 scroll = VtermScrollCommand.__new__(VtermScrollCommand)
 scroll.view = terminal.view
+# three per notch, so "one report per line" is visible
+sublime.load_settings(SETTINGS_FILE).set("wheel_lines", 3)
 
 stream.feed("\x1b[?1049h")
 scroll.run(None, up=True)