feat(plugin): guard Rust-owned classes the worker has no proxy for

The worker's autoloader fell through to the real Composer source for every
Rust-owned FQCN without a proxy stub, so plugin code doing `new Filesystem()`
or subclassing `LibraryInstaller` silently ran on a second instance the Rust
side never sees. An unimplemented part of the plugin API has to fail with an
explicit error naming it, not quietly work on a disconnected copy.

The stub generator now emits a guard class for each of those FQCNs: the real
declaration, hierarchy and constants, with every constructor and method
raising an explicit error. References satisfied by the declaration alone
(`instanceof`, `X::class`, `Link::TYPE_REQUIRE`) keep working. Two FQCNs stay
resolvable to the real class, each listed with the worker-side mechanism that
makes a natively constructed instance correct.

The error had nowhere to go: `Installer::run` dropped the `Result` of both
`dispatch_script` calls, so an exception from a listener ended in exit 0.
Both propagate now, the way the exception does upstream.

Three real-plugin E2E comparisons stop at a guard and are ignored, each
naming the class it needs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

1 files changed, 105 insertions, 0 deletions

diff --git a/crates/shirabe-php-rpc/php/guards/Composer/Downloader/FileDownloader.php b/crates/shirabe-php-rpc/php/guards/Composer/Downloader/FileDownloader.php
new file mode 100644
index 00000000..0232b002
--- /dev/null
+++ b/crates/shirabe-php-rpc/php/guards/Composer/Downloader/FileDownloader.php
@@ -0,0 +1,105 @@
+<?php
+
+// Generated by scripts/plugin-stub-generator; do not edit by hand.
+// Guard for Composer\Downloader\FileDownloader.
+// The Rust side owns this class and the worker has no proxy for it, so this
+// declaration shadows the real one: the constants and the hierarchy stay, while
+// constructing it or calling anything on it raises an explicit error.
+
+namespace Composer\Downloader;
+
+use Composer\Config;
+use Composer\Cache;
+use Composer\IO\IOInterface;
+use Composer\Package\PackageInterface;
+use Composer\EventDispatcher\EventDispatcher;
+use Composer\Util\Filesystem;
+use Composer\Util\HttpDownloader;
+use Composer\Util\ProcessExecutor;
+use React\Promise\PromiseInterface;
+
+class FileDownloader implements DownloaderInterface, ChangeReportInterface
+{
+    public static $downloadMetadata = [];
+    public static $responseHeaders = [];
+
+    public function __construct(IOInterface $io, Config $config, HttpDownloader $httpDownloader, ?EventDispatcher $eventDispatcher = null, ?Cache $cache = null, ?Filesystem $filesystem = null, ?ProcessExecutor $process = null)
+    {
+        \ShirabeUnsupportedClass::fail(self::class, '__construct');
+    }
+
+    public function getInstallationSource(): string
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'getInstallationSource');
+    }
+
+    public function download(PackageInterface $package, string $path, ?PackageInterface $prevPackage = null, bool $output = true): PromiseInterface
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'download');
+    }
+
+    public function prepare(string $type, PackageInterface $package, string $path, ?PackageInterface $prevPackage = null): PromiseInterface
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'prepare');
+    }
+
+    public function cleanup(string $type, PackageInterface $package, string $path, ?PackageInterface $prevPackage = null): PromiseInterface
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'cleanup');
+    }
+
+    public function install(PackageInterface $package, string $path, bool $output = true): PromiseInterface
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'install');
+    }
+
+    protected function getDistPath(PackageInterface $package, int $component): string
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'getDistPath');
+    }
+
+    protected function clearLastCacheWrite(PackageInterface $package): void
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'clearLastCacheWrite');
+    }
+
+    protected function addCleanupPath(PackageInterface $package, string $path): void
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'addCleanupPath');
+    }
+
+    protected function removeCleanupPath(PackageInterface $package, string $path): void
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'removeCleanupPath');
+    }
+
+    public function update(PackageInterface $initial, PackageInterface $target, string $path): PromiseInterface
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'update');
+    }
+
+    public function remove(PackageInterface $package, string $path, bool $output = true): PromiseInterface
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'remove');
+    }
+
+    protected function getFileName(PackageInterface $package, string $path): string
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'getFileName');
+    }
+
+    protected function getInstallOperationAppendix(PackageInterface $package, string $path): string
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'getInstallOperationAppendix');
+    }
+
+    protected function processUrl(PackageInterface $package, string $url): string
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'processUrl');
+    }
+
+    public function getLocalChanges(PackageInterface $package, string $path): ?string
+    {
+        \ShirabeUnsupportedClass::fail(self::class, 'getLocalChanges');
+    }
+}