feat(plugin): proxy Composer\Util\Filesystem into the plugin worker

The class was shadowed by a guard, so a plugin doing `new Filesystem()`
got an explicit error. It is classified rust-proxy and plugin-constructible
and the Rust port is complete, so listing it as a stub target and answering
its public surface from the entity is all it takes.

The constructor rejects a caller-supplied ProcessExecutor: that class has no
proxy stub, so the argument could only be a second instance the Rust side
never sees. findShortestPath re-checks its arguments at the boundary because
the port panics where PHP throws, and a panic would take the process down
instead of reaching the plugin's catch block.

phpstan/extension-installer matches upstream Composer byte for byte again.

1 files changed, 4 insertions, 0 deletions

diff --git a/crates/shirabe-php-rpc/src/lib.rs b/crates/shirabe-php-rpc/src/lib.rs
index 47304ba8..b2bc2c28 100644
--- a/crates/shirabe-php-rpc/src/lib.rs
+++ b/crates/shirabe-php-rpc/src/lib.rs
@@ -1007,6 +1007,10 @@ const STUB_FILES: &[(&str, &str)] = &[
             "../php/stubs/Composer/DependencyResolver/Operation/MarkAliasUninstalledOperation.php"
         ),
     ),
+    (
+        "Composer/Util/Filesystem.php",
+        include_str!("../php/stubs/Composer/Util/Filesystem.php"),
+    ),
 ];
 
 /// Hand-written worker-side classes (two-world implementations with behavior of their own, not