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, 0 insertions, 4 deletions

diff --git a/crates/shirabe/tests/plugin/e2e_extension_installer_test.rs b/crates/shirabe/tests/plugin/e2e_extension_installer_test.rs
index 0465757a..9a95888c 100644
--- a/crates/shirabe/tests/plugin/e2e_extension_installer_test.rs
+++ b/crates/shirabe/tests/plugin/e2e_extension_installer_test.rs
@@ -117,10 +117,6 @@ fn run_install(work: &Path, program: &str, args: &[&str]) -> InstallRun {
     }
 }
 
-// TODO(plugin): the plugin's post-install listener does `new Composer\Util\Filesystem()`, and
-// the guard class the worker loads for that FQCN raises an explicit error: the Rust side owns
-// Filesystem and has no proxy for plugin-constructed instances of it.
-#[ignore = "Filesystem is Rust-owned and has no proxy the plugin can construct; see the TODO(plugin) above"]
 #[test]
 fn test_extension_installer_install_matches_upstream_composer() {
     if !php_runtime_available() {