perf(process-executor): make execute_async genuinely concurrent

execute_async was a serial pump: it queued a job, then drove it to
completion itself via wait_id()'s blocking usleep loop before
returning, so concurrent callers never overlapped even when polled
together through FuturesUnordered.

Rewrite it as a single &self async fn mirroring the CurlDownloader/
HttpDownloader rework: a tokio Semaphore sized by max_jobs
(COMPOSER_MAX_PARALLEL_PROCESSES, PHP parity) gates admission, the
child is started non-blocking, and an async 1ms sleep loop pumps
is_running()/check_timeout() while yielding to the reactor so sibling
jobs genuinely run in parallel. The Job table, STATUS_* lifecycle,
start_job/mark_job_done/count_active_jobs/wait/wait_id all had no
remaining callers and are removed.

&self also lets callers hold only a shared borrow across their awaits
(zip_downloader, version_guesser, filesystem via the new
get_process_handle), which would otherwise panic with 'already
mutably borrowed' once two async jobs overlap on the same
Rc<RefCell<ProcessExecutor>>.

The async mock branch no longer consumes the expectation before
hitting its todo!(): the panic made that bookkeeping unobservable.

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

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