perf(installation-manager): fan out package downloads via Loop::wait

InstallationManager::downloadAndExecuteBatch now matches PHP: every
update/install operation's installer->download() promise is collected
and driven concurrently through waitOnPromises()/Loop::wait instead of
being awaited one package at a time. Concurrency caps stay where PHP
puts them (HttpDownloader 12, ProcessExecutor 10 via their semaphores).
Error semantics follow PHP too: all downloads settle before the first
rejection is rethrown, rather than aborting on the first failure.

To let the collected futures and the cleanup closures own their
installer beyond the loop iteration that created them, the installer
registry becomes Vec<Rc<dyn InstallerInterface>> and get_installer
hands out clones (PHP closures capture $installer the same way), with
InstallerInterface methods taking &self across the six implementors —
the only genuinely mutable state was LibraryInstaller.vendor_dir
(canonicalized in place), now behind a RefCell.
as_plugin_installer_mut/as_binary_presence_interface lose their &mut.

The cleanup_promises entries are now the real thing: the PHP closure
including the getInstallationSource() guard and the
installer->cleanup($opType, $package, $initialPackage) call, replacing
the no-op futures (drops one TODO(phase-b) and two TODO(phase-c)).

Verified against the real network: create-project laravel/laravel
produces a vendor tree byte-identical to real Composer's (diff -rq
clean across all 109 packages including vendor/composer).

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

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