[Server] Untangle origin tracking from Registry (#302)

Drop the isManual flag from ElementReference and the discovery-state methods
from RegistryInterface. The Registry becomes a flat last-write-wins map;
DiscoveryLoader owns its own owned-set bookkeeping so re-running discovery
only removes elements it itself contributed. Adds ChainLoader for explicit
composition. Manual-over-discovered precedence is preserved via loader
ordering, not a flag on the reference.

tests/Unit/Capability/Registry/Loader/Stub/ToolWriterLoader.php

@@ -0,0 +1,37 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Tests\Unit\Capability\Registry\Loader\Stub;
+
+use Mcp\Capability\Registry\Loader\LoaderInterface;
+use Mcp\Capability\RegistryInterface;
+use Mcp\Schema\Tool;
+
+final class ToolWriterLoader implements LoaderInterface
+{
+    public function __construct(private string $toolName, private \Closure $handler)
+    {
+    }
+
+    public function load(RegistryInterface $registry): void
+    {
+        $registry->registerTool(new Tool(
+            name: $this->toolName,
+            title: null,
+            inputSchema: ['type' => 'object', 'properties' => [], 'required' => null],
+            description: null,
+            annotations: null,
+            icons: null,
+            meta: null,
+            outputSchema: null,
+        ), $this->handler);
+    }
+}