Phase 1 sub-commit 6: skeleton wiring — make dev runs end-to-end

Symfony app under framework/skeleton/symfony/: minimal bin/console,
public/index.php, MicroKernel-based src/Kernel.php, services.yaml,
framework/security/mercure config, and a demo App\Controller\PingController
that GETs /api/ping (returning JSON pong) and republishes the same
payload to the Mercure topic app://ping. composer.json uses a path
repository to symlink the bundle from ../../php so local edits are
picked up live.

QML app under framework/skeleton/qml/: top-level CMake that
add_subdirectory's framework/qml, a main.cpp that creates the Qt
process, runs SingleInstance.acquireOrForward before any QML loads,
exposes SingleInstance via context property, and loadFromModule's
Skeleton.Main. Main.qml uses BackendConnection / RestClient /
MercureClient from PhpQml.Bridge and renders status dots, a Ping
button, and an event log.

Caddyfile binds 127.0.0.1:8765, enables in-memory Mercure with a
256-bit dev JWT (matches symfony/.env, lcobucci/jwt requires this).
Makefile wraps build / dev / doctor / clean; scripts/dev.sh starts
FrankenPHP --watch and the Qt host together with explicit PID-based
teardown (process-group `kill 0` proved unreliable when frankenphp's
watch fork reparented).

Bug fixes uncovered in this sub-commit:
- SingleInstance.acquireOrForward: probe-first, then removeServer +
  retry-listen. The original loop-with-removeServer-after-failed-bind
  silently exited on stale sockets from prior runs.
- Main.qml: MercureClient does NOT inherit BackendConnection.token —
  Mercure subscribes anonymously in dev (Caddyfile), and forwarding
  the bridge bearer made it 401-loop.
- /api/ping was 500ing because the dev MERCURE_JWT_SECRET was 144 bits;
  bumped to 64-char (>=256 bit) to satisfy lcobucci/jwt.
- Linked the framework lib (php_qml_bridge) explicitly in addition to
  the QML plugin so SingleInstance.h resolves.
- Auto-generated config/reference.php gitignored.

Smoke verified offscreen: /healthz 200, /api/ping 200, 1 publish, 1
subscriber, zero 401s, clean shutdown with no zombies.

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

framework/skeleton/symfony/.env

@@ -0,0 +1,17 @@
+APP_ENV=dev
+APP_DEBUG=1
+APP_SECRET=dev-secret-not-for-production-use
+
+# Mercure hub (FrankenPHP-built-in). Both URLs are typically the same in
+# dev mode where the hub and the app are colocated.
+MERCURE_URL=http://127.0.0.1:8765/.well-known/mercure
+MERCURE_PUBLIC_URL=http://127.0.0.1:8765/.well-known/mercure
+# Used by mercure-bundle to mint publisher JWTs. Must match the
+# publisher_jwt secret in ../Caddyfile. lcobucci/jwt requires HMAC
+# secrets to be at least 256 bits, so the dev value here is 64 chars.
+MERCURE_JWT_SECRET=dev_php_qml_bridge_jwt_secret_at_least_256_bits_long_for_lcobucci
+MERCURE_PUBLISHER_JWT_KEY=dev_php_qml_bridge_jwt_secret_at_least_256_bits_long_for_lcobucci
+MERCURE_SUBSCRIBER_JWT_KEY=dev_php_qml_bridge_jwt_secret_at_least_256_bits_long_for_lcobucci
+
+# Bearer token the Qt host sends on /api/* requests.
+BRIDGE_TOKEN=devtoken