feat(plugin): dispatch plugin event subscribers through the RPC worker

Wires the addPlugin subscriber branch end to end: EventSubscriberInterface
and Capable become fallible and dyn-compatible (their sole implementor is
the PHP plugin proxy, which answers getSubscribedEvents over RPC), listeners
register as Callable::PhpMethod and are invoked with a per-call event handle,
and the R table now drops entries when a child-side stub destructs. The R
table keeps its IndexMap with monotonically increasing handles, so released
handles are never reused and no generation counter is needed.

Upstream has no subscriber-plugin test, so the path is covered by a
Shirabe-owned fixture exercising all three getSubscribedEvents shapes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

2 files changed, 73 insertions, 0 deletions

diff --git a/crates/shirabe/tests/plugin/fixtures/subscriber-v1/Subscriber/Plugin.php b/crates/shirabe/tests/plugin/fixtures/subscriber-v1/Subscriber/Plugin.php
new file mode 100644
index 00000000..f75763ba
--- /dev/null
+++ b/crates/shirabe/tests/plugin/fixtures/subscriber-v1/Subscriber/Plugin.php
@@ -0,0 +1,61 @@
+<?php
+
+namespace Subscriber;
+
+use Composer\Composer;
+use Composer\EventDispatcher\EventSubscriberInterface;
+use Composer\IO\IOInterface;
+use Composer\Plugin\PluginInterface;
+
+class Plugin implements PluginInterface, EventSubscriberInterface
+{
+    public $version = 'subscriber-v1';
+
+    /** @var IOInterface */
+    private $io;
+
+    public function activate(Composer $composer, IOInterface $io)
+    {
+        $this->io = $io;
+        $io->write('activate subscriber-v1');
+    }
+
+    public function deactivate(Composer $composer, IOInterface $io)
+    {
+    }
+
+    public function uninstall(Composer $composer, IOInterface $io)
+    {
+    }
+
+    public static function getSubscribedEvents()
+    {
+        return [
+            'post-install-cmd' => 'onPostInstall',
+            'shirabe-priority-event' => [['early', 10], ['late', -10]],
+            'shirabe-false-event' => ['returnsFalse', 0],
+        ];
+    }
+
+    public function onPostInstall($event)
+    {
+        $this->io->write('subscriber saw ' . $event->getName());
+    }
+
+    public function early($event)
+    {
+        $this->io->write('early listener');
+    }
+
+    public function late($event)
+    {
+        $this->io->write('late listener');
+    }
+
+    public function returnsFalse($event)
+    {
+        $this->io->write('failing listener');
+
+        return false;
+    }
+}
diff --git a/crates/shirabe/tests/plugin/fixtures/subscriber-v1/composer.json b/crates/shirabe/tests/plugin/fixtures/subscriber-v1/composer.json
new file mode 100644
index 00000000..40659606
--- /dev/null
+++ b/crates/shirabe/tests/plugin/fixtures/subscriber-v1/composer.json
@@ -0,0 +1,12 @@
+{
+    "name": "subscriber-v1",
+    "version": "1.0.0",
+    "type": "composer-plugin",
+    "autoload": { "psr-0": { "Subscriber": "" } },
+    "extra": {
+        "class": "Subscriber\\Plugin"
+    },
+    "require": {
+        "composer-plugin-api": "^2.0"
+    }
+}