tui: port framework core (style, theme, input, VT screen, compositor, panel, widgets, app, demo); demo frame byte-identical to tai.py

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

taicpp/tests/tui_shot.cpp

@@ -0,0 +1,44 @@
+// tui_shot: headless frames of the demo scene from the C++ port, to diff against python.
+//   tui_shot [rows cols]            plain text, same as tai.py --demo-shot
+//   tui_shot frame [rows cols]      the raw bytes of the first frame (TuiApp.frame_bytes)
+//   tui_shot cells [rows cols]      one line per cell: y x codepoint sgr (compose, no diff)
+#include "TuiDemo.h"
+#include <cstdio>
+#include <cstdlib>
+#include <iostream>
+#include <string>
+using namespace detz::tui;
+
+int main(int argc, char** argv) {
+    std::string mode = "text";
+    int arg = 1;
+    if (argc > 1 && (std::string(argv[1]) == "frame" || std::string(argv[1]) == "cells")) {
+        mode = argv[1];
+        arg = 2;
+    }
+    int rows = argc > arg ? std::atoi(argv[arg]) : 30;
+    int cols = argc > arg + 1 ? std::atoi(argv[arg + 1]) : 100;
+    if (mode == "text") return TuiDemo::demo_shot(std::cout, rows, cols);
+    TuiApp app;
+    app.rows = rows;
+    app.cols = cols;
+    auto built = TuiDemo::build(app, false);
+    app.mount(built.first);
+    app.close_panel(built.first->find("welcome"));
+    built.second->shell->screen.feed("$ echo hello\r\nhello\r\n$ ");
+    built.second->stream->screen.feed("stream tick 1\r\nstream tick 2\r\nstream tick 3\r\n");
+    if (mode == "frame") {
+        std::string bytes = app.frame_bytes();
+        std::fwrite(bytes.data(), 1, bytes.size(), stdout);
+        return 0;
+    }
+    app.render_text();   // forces layout
+    TuiCellRows frame = app.compositor.compose(app.root.get(), rows, cols);
+    for (size_t y = 0; y < frame.size(); y++) {
+        for (size_t x = 0; x < frame[y].size(); x++) {
+            std::printf("%zu %zu %u %s\n", y, x, static_cast<unsigned>(frame[y][x].ch),
+                        app.compositor.sgr(frame[y][x].key).substr(1).c_str());
+        }
+    }
+    return 0;
+}