Fiber-native concurrent application pipeline #357

Area: Support\Router, HTTP, react/event-loop

Problem

Doppar depends on React's event loop and has stream-oriented responses, but
application code has no framework-level way to run independent I/O tasks with
structured cancellation and deadlines.

Proposal

Add a small TaskGroup API built on PHP Fibers. Tasks inherit the current
execution context, support a shared deadline, and are cancelled together when
one task fails. The API must remain opt-in so ordinary synchronous controllers
work unchanged.

Acceptance criteria

• A task group can run independent tasks concurrently on the configured loop.
• Exceptions include the failed task name and cancel sibling tasks.
• Deadlines and cancellation are deterministic in tests.
• No request can finish while child tasks remain registered.