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/Caddyfile

@@ -0,0 +1,28 @@
+{
+	auto_https off
+	admin off
+	frankenphp
+	order php_server before respond
+	order mercure after encode
+}
+
+http://127.0.0.1:8765 {
+	root * php/
+	encode gzip
+
+	mercure {
+		transport local
+		publisher_jwt !ChangeThisDevKey!
+		subscriber_jwt !ChangeThisDevKey!
+		anonymous
+		publish_origins *
+		cors_origins *
+	}
+
+	php_server
+
+	log {
+		output stderr
+		format console
+	}
+}