refactor(downloader): take &self across the downloader hierarchy

Concurrent package operations call into the same downloader instances
through Rc<RefCell<dyn DownloaderInterface>>; with &mut self methods
every call holds a RefMut across its awaits, which panics with
'already mutably borrowed' the moment two operations overlap. This is
groundwork for fanning out InstallationManager's download/install
loops (same rework HttpDownloader/CurlDownloader already got).

- DownloaderInterface/ChangeReportInterface/ArchiveDownloader/
  VcsDownloader methods now take &self; as_change_report_interface
  returns &dyn instead of &mut dyn.
- Implementors move their genuinely mutable state behind cells:
  FileDownloader.additional_cleanup_paths, the archive downloaders'
  cleanup_executed, ZipDownloader.zip_archive_object,
  VcsDownloaderBase.has_cleaned_changes, GitDownloader's stash/discard/
  cache maps and GitUtil, SvnDownloader.cache_credentials,
  PerforceDownloader.perforce. FileDownloader.io gains a RefCell layer
  so get_local_changes can keep PHP's NullIO swap under &self.
- ProcessExecutor::execute_async now returns a future that captures
  everything up front instead of borrowing the executor, and call
  sites build the future before awaiting, so no borrow on the shared
  executor is held while a subprocess runs.
- Filesystem::remove_directory_async becomes remove_directory_async_via
  taking the Rc handle: the Filesystem is only borrowed for the sync
  head/tail, never across the rm subprocess await (sync borrow_mut
  users like rename/ensure_directory_exists would otherwise collide).
- DownloadManager async call sites hold shared borrows only.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/crates/shirabe/tests/downloader/perforce_downloader_test.rs b/crates/shirabe/tests/downloader/perforce_downloader_test.rs
index bdf24a2e..c657e244 100644
--- a/crates/shirabe/tests/downloader/perforce_downloader_test.rs
+++ b/crates/shirabe/tests/downloader/perforce_downloader_test.rs
@@ -83,7 +83,7 @@ fn test_init_perforce_instantiates_a_new_perforce_object() {
     let (process, _process_guard) =
         get_process_executor_mock(vec![], false, MockHandler::default());
     let fs = std::rc::Rc::new(std::cell::RefCell::new(Filesystem::new(None)));
-    let mut downloader = PerforceDownloader::new(io, config, process, fs);
+    let downloader = PerforceDownloader::new(io, config, process, fs);
 
     downloader.init_perforce(
         package,
@@ -101,7 +101,7 @@ fn test_init_perforce_does_nothing_if_perforce_already_set() {
     let (process, _process_guard) =
         get_process_executor_mock(vec![], false, MockHandler::default());
     let fs = std::rc::Rc::new(std::cell::RefCell::new(Filesystem::new(None)));
-    let mut downloader = PerforceDownloader::new(io, config, process, fs);
+    let downloader = PerforceDownloader::new(io, config, process, fs);
 
     // The already-set perforce only sees initializePath; the repository's getRepoConfig is
     // never consulted (the early return happens before reaching it).
@@ -146,7 +146,7 @@ fn do_install_workflow(source_ref: &'static str, expected_label: Option<String>)
     let (process, _process_guard) =
         get_process_executor_mock(vec![], false, MockHandler::default());
     let fs = std::rc::Rc::new(std::cell::RefCell::new(Filesystem::new(None)));
-    let mut downloader = PerforceDownloader::new(io, config, process, fs);
+    let downloader = PerforceDownloader::new(io, config, process, fs);
 
     let mut perforce = MockPerforce::new();
     let expected_path = test_path_str.clone();