Phase 1 sub-commit 4: Qt foundation types

BackendConnection (QML singleton via create() factory) reads BRIDGE_URL
and BRIDGE_TOKEN from env, periodically probes <url>/healthz with a 2s
transfer timeout, and exposes a Connecting/Online/Error state machine
plus error/token properties to QML. Bundled-mode startup (spawning the
embedded FrankenPHP child) is a Phase 4 deliverable; restart() is a
no-op for now. tokenRotated signal is reserved for the per-session
secret rotation described in PLAN.md §3.

SingleInstance is C++-only — main() must call acquireOrForward() before
the QML engine boots, so it's exposed via context property rather than
QML_SINGLETON. QLocalServer-based lock with stale-socket detection,
launch-arg forwarding via QDataStream, and the deadlock-avoiding race
fallback specified in §3 *Edge cases*.

CMakeLists.txt declares the PhpQml.Bridge static QML module with both
sources and is dual-mode: stands alone for sanity builds, integrates
via add_subdirectory from the skeleton's top-level CMake (Phase 1
sub-commit 6). Standalone build verified clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

framework/qml/src/SingleInstance.h

@@ -0,0 +1,47 @@
+#pragma once
+
+#include <QLocalServer>
+#include <QObject>
+#include <QString>
+#include <QStringList>
+
+namespace PhpQml::Bridge {
+
+/// Per-OS-user single-instance lock with launch-arg forwarding.
+///
+/// Owned by the application's `main()`, NOT a QML singleton — the
+/// acquire-or-forward decision must run before the QML engine boots,
+/// so we cannot rely on lazy QML construction. The application exposes
+/// the live instance to QML via `setContextProperty("SingleInstance", &si)`.
+///
+/// See PLAN.md §3 (Single-instance, Edge cases — Single-instance launch race).
+class SingleInstance : public QObject
+{
+    Q_OBJECT
+
+public:
+    explicit SingleInstance(const QString& applicationId, QObject* parent = nullptr);
+
+    /// Returns true if this process is the live instance and should
+    /// continue starting up. Returns false if another instance was
+    /// already running; the launch arguments have been forwarded and
+    /// the caller must exit before creating any QML/window resources.
+    bool acquireOrForward(const QStringList& launchArgs);
+
+signals:
+    /// Emitted when a subsequent invocation forwards its launch args
+    /// to the running instance. Application code is expected to act
+    /// on this — typically focus the existing window or open a new one.
+    void launchArgsReceived(const QStringList& args);
+
+private slots:
+    void onNewConnection();
+
+private:
+    QString endpointName() const;
+
+    QString m_appId;
+    QLocalServer m_server;
+};
+
+} // namespace PhpQml::Bridge