feat(0132): add terminal_panel demo (oneshot, interactive, readonly)

Add demos_terminal.cpp con 3 demos del modulo terminal_panel:
- Demo 1: one-shot spawn bash, echo hello; date; ls /tmp
- Demo 2: terminal interactivo bash/cmd.exe con input box
- Demo 3: readonly tail -f con boton append

Actualiza CMakeLists.txt con ansi_parser + terminal_panel + -lutil (Linux).
Actualiza demos.h + main.cpp con entry "Terminal" en el sidebar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

demos.h

@@ -53,4 +53,7 @@ void demo_shader_canvas();
 void demo_gl_texture();       // wave 1, issue 0026
 void demo_gl_info();          // issue 0049b — runtime GL version + 4.3 caps
 
+// --- Terminal (issue 0132) ---
+void demo_terminal();
+
 } // namespace gallery