fix(terminal): Fix vertical jumpiness by joining buffer lines with newlines

The terminal display was using  which concatenated lines
without separators, causing text to render incorrectly and appear to jump
vertically.

Changed to  so lines are properly separated with
newlines, allowing the terminal to render each line on its own row.

The ANSI cursor positioning codes (\033[row;colH) added by effects like
HUD and firehose still work correctly because:
1. \033[H moves cursor to (1,1) and \033[J clears screen
2. Newlines move cursor down for subsequent lines
3. Cursor positioning codes override the newline positions
4. This allows effects to position content at specific rows while the
   base content flows naturally with newlines

engine/display/backends/terminal.py

@@ -110,7 +110,7 @@ class TerminalDisplay:
             buffer = render_border(buffer, self.width, self.height, fps, frame_time)
 
         # Write buffer with cursor home + erase down to avoid flicker
-        output = "\033[H\033[J" + "".join(buffer)
+        output = "\033[H\033[J" + "\n".join(buffer)
         sys.stdout.buffer.write(output.encode())
         sys.stdout.flush()