Add Phase 0 spike: end-to-end transport verified

Bare PHP behind FrankenPHP plus a Qt/QML host that spawns it. GET /api/ping
publishes a Mercure event; the QML window receives it back over the SSE
stream. Findings (Caddy directive ordering, Mercure transport scalar,
PR_SET_PDEATHSIG for child cleanup, PHP 8.5 curl_close deprecation, port
collision with system FrankenPHP, pure-QML SSE viability) are recorded in
spike/README.md so Phase 1 starts from a known-good baseline.

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

spike/qt/main.cpp

@@ -0,0 +1,72 @@
+#include <QCoreApplication>
+#include <QGuiApplication>
+#include <QQmlApplicationEngine>
+#include <QProcess>
+#include <QDir>
+#include <QFileInfo>
+#include <QObject>
+#include <QDebug>
+
+#include <csignal>
+#include <sys/prctl.h>
+
+// Resolve the spike root by walking up from the running binary
+// until we find a directory containing Caddyfile + bin/frankenphp.
+static QString resolveSpikeRoot()
+{
+    QDir d(QCoreApplication::applicationDirPath());
+    for (int i = 0; i < 6; ++i) {
+        if (QFileInfo(d.filePath("Caddyfile")).exists() &&
+            QFileInfo(d.filePath("bin/frankenphp")).exists()) {
+            return d.absolutePath();
+        }
+        if (!d.cdUp()) break;
+    }
+    qWarning() << "Could not locate spike root from"
+               << QCoreApplication::applicationDirPath();
+    return QCoreApplication::applicationDirPath();
+}
+
+// SIGTERM / SIGINT → graceful Qt quit so aboutToQuit cleanup runs.
+static void installSignalHandlers()
+{
+    auto handler = +[](int) { QCoreApplication::quit(); };
+    std::signal(SIGTERM, handler);
+    std::signal(SIGINT,  handler);
+}
+
+int main(int argc, char *argv[])
+{
+    QGuiApplication app(argc, argv);
+    installSignalHandlers();
+
+    const QString spikeRoot = resolveSpikeRoot();
+    qInfo() << "spike root:" << spikeRoot;
+
+    QProcess franken;
+    franken.setProgram(spikeRoot + "/bin/frankenphp");
+    franken.setArguments({"run", "--config", spikeRoot + "/Caddyfile"});
+    franken.setWorkingDirectory(spikeRoot);
+    franken.setProcessChannelMode(QProcess::ForwardedChannels);
+
+    // Linux belt-and-suspenders: ask the kernel to SIGTERM the child
+    // when this process dies, regardless of how (crash, kill -9, etc.).
+    franken.setChildProcessModifier([] {
+        prctl(PR_SET_PDEATHSIG, SIGTERM);
+    });
+
+    franken.start();
+
+    QObject::connect(&app, &QCoreApplication::aboutToQuit, [&]() {
+        if (franken.state() == QProcess::NotRunning) return;
+        franken.terminate();
+        if (!franken.waitForFinished(2000)) franken.kill();
+    });
+
+    QQmlApplicationEngine engine;
+    engine.loadFromModule("Spike", "Main");
+
+    if (engine.rootObjects().isEmpty()) return -1;
+
+    return app.exec();
+}