Phase 1 sub-commit 2: Symfony bundle internals

Bundle code for php-qml/bridge: BridgeBundle (AbstractBundle, autoloads
config/services.yaml), Publisher (thin wrapper over Mercure HubInterface
that enforces envelope-as-JSON), SessionAuthenticator (bearer-token
custom Symfony authenticator with problem+json failures), and
HealthController (GET /healthz readiness probe).

Composer constraints bumped to Symfony ^8.0 across the board (per user
request); mercure component to ^0.7. PHPUnit 11 suite covers Publisher
publish + private flag and SessionAuthenticator support/auth/failure
paths — 8 tests, 22 assertions, all green.

PLAN.md §13 updated to record the Symfony 8 minimum.

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

framework/php/src/Publisher.php

@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpQml\Bridge;
+
+use Symfony\Component\Mercure\HubInterface;
+use Symfony\Component\Mercure\Update;
+
+/**
+ * Publishes envelopes onto the bridge's Mercure hub.
+ *
+ * Topic conventions and envelope shape are defined in PLAN.md §4.
+ * Reactive-model-aware helpers (publishModelUpdate, etc.) arrive with
+ * the model layer in Phase 2.
+ */
+final readonly class Publisher
+{
+    public function __construct(
+        private HubInterface $hub,
+    ) {
+    }
+
+    /**
+     * @param array<string, mixed> $envelope
+     */
+    public function publish(string $topic, array $envelope, bool $private = false): string
+    {
+        return $this->hub->publish(new Update(
+            $topic,
+            json_encode($envelope, JSON_THROW_ON_ERROR),
+            $private,
+        ));
+    }
+}