feat(installer): wire InstallerInterface marker-trait downcasts

Add as_binary_presence_interface and as_plugin_installer_mut to
InstallerInterface following the downloader marker-trait downcast
pattern, so InstallationManager can model the PHP instanceof checks in
ensureBinariesPresence and disablePlugins. Make BinaryPresenceInterface
take &mut self, resolving LibraryInstaller's stubbed trait impl.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 6 insertions, 5 deletions

diff --git a/crates/shirabe/src/installer/library_installer.rs b/crates/shirabe/src/installer/library_installer.rs
index b4b2d20..58706d6 100644
--- a/crates/shirabe/src/installer/library_installer.rs
+++ b/crates/shirabe/src/installer/library_installer.rs
@@ -418,13 +418,14 @@ impl InstallerInterface for LibraryInstaller {
             base_path
         })
     }
+
+    fn as_binary_presence_interface(&mut self) -> Option<&mut dyn BinaryPresenceInterface> {
+        Some(self)
+    }
 }
 
 impl BinaryPresenceInterface for LibraryInstaller {
-    fn ensure_binaries_presence(&self, _package: PackageInterfaceHandle) {
-        // TODO(phase-b): trait takes &self but LibraryInstaller::ensure_binaries_presence
-        // requires &mut self due to BinaryInstaller::install_binaries(&mut self, ...).
-        // Revisit the trait or use interior mutability.
-        todo!()
+    fn ensure_binaries_presence(&mut self, package: PackageInterfaceHandle) {
+        LibraryInstaller::ensure_binaries_presence(self, package);
     }
 }