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_terminal.cpp

@@ -0,0 +1,187 @@
+// demos_terminal.cpp — demos del modulo terminal_panel (issue 0132).
+//
+// Demo 1: One-shot — spawn bash que ejecuta "echo hello; date; ls /tmp | head"
+//         con auto-close cuando el proceso termina. Muestra el scrollback.
+// Demo 2: Interactivo — bash -i (Linux) / cmd.exe (Windows) con input box.
+// Demo 3: Readonly — tail simulado de un archivo de log.
+
+#include "demos.h"
+#include "demo.h"
+#include "viz/terminal_panel/terminal_panel.h"
+#include "imgui.h"
+
+#include <cstring>
+#include <string>
+
+namespace gallery {
+
+// ---------------------------------------------------------------------------
+// Demo 1: One-shot con auto-close
+// ---------------------------------------------------------------------------
+
+void demo_terminal_oneshot() {
+    demo_header("terminal_panel", "v1.0.0",
+        "Emulador TTY embebible. Demo 1: one-shot — spawn bash, ejecuta comandos, auto-close.");
+
+    static fn_term::TerminalPanel s_term;
+    static bool s_started = false;
+
+    if (!s_started) {
+        ImGui::TextUnformatted("Pulsa 'Run' para lanzar el one-shot.");
+        if (ImGui::Button("Run")) {
+            if (!s_term.is_open()) {
+                s_term.shell = "/bin/bash";
+                s_term.scrollback_lines = 200;
+                s_term.readonly = true; // solo output, sin input
+                fn_term::open(s_term);
+                fn_term::send(s_term, "echo hello; date; ls /tmp | head -5; exit 0\n");
+                s_started = true;
+            }
+        }
+        return;
+    }
+
+    fn_term::render(s_term);
+
+    if (s_term.process_exited.load() && s_term.is_open()) {
+        ImGui::SameLine();
+        ImGui::TextDisabled("[done — exit %d]", s_term.exit_code);
+    }
+
+    if (ImGui::Button("Reset demo")) {
+        if (s_term.is_open()) fn_term::close(s_term);
+        s_started = false;
+    }
+
+    code_block(
+        "fn_term::TerminalPanel term;\n"
+        "term.shell    = \"/bin/bash\";\n"
+        "term.readonly = true;\n"
+        "fn_term::open(term);\n"
+        "fn_term::send(term, \"echo hello; date; exit 0\\n\");\n"
+        "// En cada frame ImGui:\n"
+        "fn_term::render(term);\n"
+        "if (term.process_exited.load()) fn_term::close(term);"
+    );
+}
+
+// ---------------------------------------------------------------------------
+// Demo 2: Interactivo
+// ---------------------------------------------------------------------------
+
+void demo_terminal_interactive() {
+    demo_header("terminal_panel", "v1.0.0",
+        "Demo 2: terminal interactivo — bash -i (Linux) / cmd.exe (Windows).");
+
+    static fn_term::TerminalPanel s_term;
+
+    if (!s_term.is_open()) {
+        if (ImGui::Button("Open shell")) {
+#ifdef _WIN32
+            s_term.shell = "cmd.exe";
+#else
+            s_term.shell = "/bin/bash";
+#endif
+            s_term.scrollback_lines = 1000;
+            s_term.readonly = false;
+            fn_term::open(s_term);
+        }
+        return;
+    }
+
+    fn_term::render(s_term);
+
+    if (ImGui::Button("Close shell")) {
+        fn_term::close(s_term);
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Demo 3: Readonly tail
+// ---------------------------------------------------------------------------
+
+void demo_terminal_readonly() {
+    demo_header("terminal_panel", "v1.0.0",
+        "Demo 3: readonly — tail de /tmp/fn_gallery_test.log (creado al abrir).");
+
+    static fn_term::TerminalPanel s_term;
+    static bool s_log_created = false;
+
+    // Crear el archivo de log de prueba si no existe.
+    if (!s_log_created) {
+        if (ImGui::Button("Start tail")) {
+            // Escribir unas líneas de muestra al log.
+            FILE* f = fopen("/tmp/fn_gallery_test.log", "w");
+            if (f) {
+                for (int i = 0; i < 5; i++)
+                    fprintf(f, "[sample] log line %d\n", i);
+                fclose(f);
+            }
+            s_term.shell    = "/bin/bash";
+            s_term.readonly = true;
+            s_term.scrollback_lines = 500;
+            fn_term::open(s_term);
+            fn_term::send(s_term, "tail -f /tmp/fn_gallery_test.log\n");
+            s_log_created = true;
+        }
+        return;
+    }
+
+    fn_term::render(s_term);
+
+    ImGui::Separator();
+    if (ImGui::Button("Append line")) {
+        FILE* f = fopen("/tmp/fn_gallery_test.log", "a");
+        if (f) {
+            static int s_count = 0;
+            fprintf(f, "[appended] line %d\n", s_count++);
+            fclose(f);
+        }
+    }
+    ImGui::SameLine();
+    if (ImGui::Button("Stop")) {
+        if (s_term.is_open()) fn_term::close(s_term);
+        s_log_created = false;
+    }
+
+    code_block(
+        "fn_term::TerminalPanel term;\n"
+        "term.shell    = \"/bin/bash\";\n"
+        "term.readonly = true;          // sin input box\n"
+        "fn_term::open(term);\n"
+        "fn_term::send(term, \"tail -f /tmp/agent.log\\n\");\n"
+        "// En cada frame:\n"
+        "fn_term::render(term);"
+    );
+}
+
+// ---------------------------------------------------------------------------
+// Entry point unificado (llamado desde main.cpp via demos.h)
+// ---------------------------------------------------------------------------
+
+void demo_terminal() {
+    static int s_tab = 0;
+    if (ImGui::BeginTabBar("##term_tabs")) {
+        if (ImGui::BeginTabItem("One-shot")) {
+            s_tab = 0;
+            ImGui::EndTabItem();
+        }
+        if (ImGui::BeginTabItem("Interactive")) {
+            s_tab = 1;
+            ImGui::EndTabItem();
+        }
+        if (ImGui::BeginTabItem("Readonly tail")) {
+            s_tab = 2;
+            ImGui::EndTabItem();
+        }
+        ImGui::EndTabBar();
+    }
+
+    switch (s_tab) {
+    case 0: demo_terminal_oneshot();     break;
+    case 1: demo_terminal_interactive(); break;
+    case 2: demo_terminal_readonly();    break;
+    }
+}
+
+} // namespace gallery