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, 2 insertions, 2 deletions

diff --git a/crates/shirabe/src/installer.rs b/crates/shirabe/src/installer.rs
index 9b4a84e8..ee60f59f 100644
--- a/crates/shirabe/src/installer.rs
+++ b/crates/shirabe/src/installer.rs
@@ -273,7 +273,7 @@ impl Installer {
                 self.dev_mode,
                 vec![],
                 IndexMap::new(),
-            );
+            )?;
         }
 
         self.download_manager
@@ -469,7 +469,7 @@ impl Installer {
                 self.dev_mode,
                 vec![],
                 IndexMap::new(),
-            );
+            )?;
         }
 
         let audit_config = self.get_audit_config()?.clone();