feat(qbz-qt): phase 0 — crate skeleton + cxx-qt toolchain gate

New qbz-qt bin crate (cxx-qt 0.7 + QML) as the only default workspace
member in this worktree, so plain cargo build produces just the Qt
binary. Gate verified against system Qt 6.11.1 (codegen, moc,
qmltyperegistrar, qrc load, offscreen run all green).

km

crates/qbz-qt/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "qbz-qt"
+description = "Qt/QML frontend POC for QBZ (cxx-qt bridge over the UI-agnostic backend)"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[[bin]]
+name = "qbz-qt"
+path = "src/main.rs"
+
+[dependencies]
+cxx = "1"
+cxx-qt = "0.7"
+cxx-qt-lib = { version = "0.7", features = ["qt_full"] }
+
+[build-dependencies]
+cxx-qt-build = "0.7"

crates/qbz-qt/build.rs

@@ -0,0 +1,16 @@
+use cxx_qt_build::{CxxQtBuilder, QmlModule};
+
+fn main() {
+    CxxQtBuilder::new()
+        // Qt modules the bridge links against (Qt6 CMake in /usr/lib64/cmake).
+        .qt_module("Qml")
+        .qt_module("Quick")
+        .qt_module("QuickControls2")
+        .qml_module(QmlModule {
+            uri: "com.blitzfc.qbz",
+            rust_files: &["src/bridge.rs"],
+            qml_files: &["qml/Main.qml"],
+            ..Default::default()
+        })
+        .build();
+}

crates/qbz-qt/qml/Main.qml

@@ -0,0 +1,21 @@
+import QtQuick
+import QtQuick.Controls
+
+ApplicationWindow {
+    visible: true
+    width: 960
+    height: 600
+    title: "qbz-qt — toolchain gate"
+
+    Rectangle {
+        anchors.fill: parent
+        color: "#191922"
+
+        Text {
+            anchors.centerIn: parent
+            text: "qbz-qt phase 0 gate"
+            color: "#e8e8ef"
+            font.pixelSize: 20
+        }
+    }
+}

crates/qbz-qt/src/bridge.rs

@@ -0,0 +1,25 @@
+//! Phase 0 toolchain-gate bridge.
+//!
+//! A single trivial QObject proves the cxx-qt codegen <-> Qt 6.11 pipeline
+//! (qmltyperegistrar, moc, CMake link) before any real binding work lands.
+
+#[cxx_qt::bridge]
+mod qbz_bridge {
+    extern "RustQt" {
+        #[qobject]
+        #[qml_element]
+        #[qproperty(QString, greeting)]
+        type HelloBridge = super::HelloBridgeRust;
+    }
+
+    extern "C++" {
+        include!("cxx-qt-lib/qstring.h");
+        type QString = cxx_qt_lib::QString;
+    }
+}
+
+/// Rust side of the phase-0 object (constructed from QML if instantiated).
+#[derive(Default)]
+pub struct HelloBridgeRust {
+    greeting: cxx_qt_lib::QString,
+}

crates/qbz-qt/src/main.rs

@@ -0,0 +1,22 @@
+//! qbz-qt — Qt/QML frontend POC entry point.
+//!
+//! Phase 0: boot a QML engine on the qrc module and show a gate window.
+
+mod bridge;
+
+use cxx_qt_lib::{QGuiApplication, QQmlApplicationEngine, QUrl};
+
+fn main() {
+    let mut app = QGuiApplication::new();
+    let mut engine = QQmlApplicationEngine::new();
+
+    if let Some(engine) = engine.as_mut() {
+        engine.load(&QUrl::from(
+            "qrc:/qt/qml/com/blitzfc/qbz/qml/Main.qml",
+        ));
+    }
+
+    if let Some(app) = app.as_mut() {
+        app.exec();
+    }
+}