perf(filesystem): delete directories without spawning rm -rf

Composer shells out to `rm -rf` because PHP has no recursive directory removal.
Rust has one, and every package installed from a dist archive pays for a removal:
the extraction pipeline drops its temporary directory once per package, and
uninstalling a package removes its whole tree.

The asynchronous path goes through `tokio::fs` rather than `std::fs`, so the walk
runs on a blocking thread and the sibling installs the reactor is driving keep
making progress, the way they did while the subprocess was working.

Installing laravel/laravel (109 packages) from a warm cache drops from 3.85 to
3.19 CPU seconds. The removals run concurrently, so on an idle 16-core machine
they never reach the critical path and wall time is unchanged at 1.65 s; pinned
to two cores it falls from 2.33 s to 2.20 s. Pruning the 33 dev packages with
`install --no-dev`, where whole package trees are removed rather than empty
temporary directories, drops from 844 ms to 806 ms even on 16 cores.

Windows keeps the `rmdir /S /Q` subprocess, and both platforms keep falling back
to `remove_directory_php` when the fast path does not clear the directory.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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