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/CMakeLists.txt

@@ -0,0 +1,29 @@
+cmake_minimum_required(VERSION 3.21)
+project(php_qml_spike LANGUAGES CXX)
+
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_AUTOMOC ON)
+
+find_package(Qt6 6.5 REQUIRED COMPONENTS Core Gui Quick QuickControls2 Network Qml)
+
+qt_standard_project_setup(REQUIRES 6.5)
+
+qt_add_executable(spike main.cpp)
+
+qt_add_qml_module(spike
+    URI Spike
+    VERSION 1.0
+    QML_FILES
+        Main.qml
+        Mercure.qml
+)
+
+target_link_libraries(spike PRIVATE
+    Qt6::Core
+    Qt6::Gui
+    Qt6::Quick
+    Qt6::QuickControls2
+    Qt6::Network
+    Qt6::Qml
+)