Add ControlledTestKit for simulated-time actor tests

[nb-ceffa]

Summary

Adds a new cross-platform cats-actors-testkit module that lets you test actors under simulated time using Cats Effect's TestControl. Time-dependent behaviour (schedulers, receive timeouts,
retries) can be tested deterministically and instantly — no real-world sleeps, no flaky timing thresholds. Also includes a few quality-of-life improvements to the existing TestKit.

Motivation

Actor tests that involve delays currently have to sleep in real time, which makes suites slow and timing-sensitive. Cats Effect provides TestControl to drive an IO program over a simulated clock,
but wiring an ActorSystem into it correctly (execute → tick → interpret outcome) is fiddly boilerplate. This PR packages that pattern into a reusable, framework-agnostic helper.

What's included

• ControlledTestKit — a trait (extending TestKit) exposing withActorSystemIO, which:
  • provisions an ActorSystem[IO],
  • runs your test body inside TestControl,
  • advances the simulated clock (testTimeWindow, default 90s),
  • and interprets the outcome.
• ControlledTestException — a small sealed hierarchy (Canceled, TimedOut(window)) for the non-success outcomes that don't originate from the test logic itself.
• TestKit improvements (existing module):
  • new expectMsgTypeSingle / expectMsgTypeCountN helpers (count-based, snapshot-independent message-type assertions),
  • the failing-expectMsg path now logs the current queue state to aid debugging,
  • extended Scaladoc on expectMsgType documenting the snapshot/splitAt race that can make it flaky, with a pointer to the more robust count-based helpers.
• Build wiring — new testkit cross-project (JVM / JS / Native), aggregated under root.
• README — a new "Testing Actors" section with a runnable example.

Backward compatibility

Fully additive. The only change to existing code is in TestKit (new methods + extra logging + Scaladoc); no signatures changed and no behaviour removed.