Add multi-callback worker pool API to ForkManager

[coisa]

Context

The current ForkManagerInterface::fork(callable $workerCallback, int $workerCount = 1) (and implementation) only accepts a single callback and spawns N workers for that same callback.

This is limiting:

• we can only scale the same unit of work;
• there is no way to define different routines per worker in the same call.

Why this matters

In practice, if a caller needs worker types like:

• one process validating input,
• one process doing IO-bound work,
• one process handling notifications,

it currently requires multiple fork() calls and orchestration outside the component, or custom dispatch logic.

Proposed change

Add support for multi-callback worker pools so one fork orchestration can map each callback independently.

Recommended API (non-breaking first)

Keep existing behavior and add:

• forkMany(array $workerCallbacks) to dispatch a heterogeneous callback set.

Proposed behavior:

• key/index in the array identifies each worker callback task;
• each callback receives the same shared context as today (or explicit context strategy if needed);
• result/exception collection remains deterministic per worker.

Alternative (possibly breaking)

Replace fork(callable $workerCallback, int $workerCount = 1) with a variadic/callback-list signature.

Because this is BC-sensitive, this should be considered carefully.

Non-goals

• changing the fork strategy/scheduler itself;
• adding process-pool abstractions or advanced load-balancing in this first iteration.

Acceptance criteria

• New tests covering:
  • mixed callbacks executed in the same fork invocation;
  • worker count aligned with callback list length;
  • independent callback failure reporting.
• Keep old fork(callable, int) behavior untouched when using the legacy API.