Thread-spawning primitive provided by runtimes that support worker threads.
Absent on runtimes without them (e.g. Cloudflare Workers) — consumers MUST
degrade gracefully when it is not provided (see the WorkerPoolPlugin, which
fails run() with a typed error when no host exists).
spawn(specifier: string): IWorkerHandle
Spawns a module worker.
availableParallelism(): number
Number of threads the host can usefully run in parallel.
reportsExit(): boolean
Reports whether handles from spawn will implement
IWorkerHandle.onExit.
Answerable BEFORE a worker exists, which is the reason it sits here rather than being inferred from a handle: a consumer that must warn about — or configure around — undetectable worker death has to know at registration time, and the first spawn may be far later or never happen at all.
Optional so an existing custom host stays valid; a host that omits it is
treated exactly as one reporting false.