fix(statechart, overmind): await async exit actions, fix devtools actions, fix throttle stale value

[ericsorenson]

Summary

• Fix statechart exit actions not being awaited when async, breaking exit → action → transition → entry ordering
• Fix devtools incorrectly showing onInitializeOvermind as a transition action after state changes
• Fix throttle operator passing the first invocation's value instead of the most recent one

Bug Fixes

1. Statechart: async exit actions not awaited before transitions

When a statechart exit action was async (plain async function or operator-based via pipe), the returned
Promise was discarded. The main action, state transition, and entry actions executed immediately without
waiting for exit to complete.

The runAction wrapper now collects exit action return values, detects Promises via isPromise, and
Promise.all()s them before proceeding. After async exits complete, it re-validates the transition is
still valid since another action may have changed state during the async gap. The synchronous code path
is unchanged.

2. Statechart: wrong actions object passed to devtools

After a state transition, the devtools update used config.actions (which includes onInitializeOvermind)
instead of copiedActions (which has it removed). The other two call sites already used copiedActions
correctly.

3. Throttle operator passes stale value

The throttle operator's setTimeout closure captured value from the first invocation. Subsequent calls
updated currentNext but not the value inside the already-scheduled timeout. Added a currentValue
variable updated on every invocation so the timeout fires with the most recent value.

Other Changes

• Exported isPromise from overmind's public API so overmind-statechart can import it instead of
  duplicating the implementation
• Added 6 new statechart tests covering operator-based actions and async exit ordering
• Added 1 new throttle test verifying the most recent value is passed downstream

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[henri-hulski]

Great work! Good you take a look at statecharts, another package which didn't get much care.

BTW if you fix different task in a PR you can use several commits. One commit is one entry in the release notes (which replaces the changelog). So for every entry in the release notes you need to create a separate commit.