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>

main.cpp

@@ -81,6 +81,8 @@ static const DemoEntry k_demos[] = {
     {"shader_canvas",   "shader_canvas",   "Gfx",  &gallery::demo_shader_canvas},
     {"gl_texture",      "gl_texture_load", "Gfx",  &gallery::demo_gl_texture},      // wave 1
     {"gl_info",         "gl_info",         "Gfx",  &gallery::demo_gl_info},         // issue 0049b
+    // Terminal (issue 0132)
+    {"terminal_panel",  "terminal_panel",  "Terminal", &gallery::demo_terminal},
 };
 static constexpr int k_demo_count = sizeof(k_demos) / sizeof(k_demos[0]);