Fix Collection was modified crash in ManualMotionDispatcher when scheduling during Update

[Sahasrara]

Problem

ManualMotionDispatcher.Update and Reset enumerate the runners dictionary directly:

foreach (var kv in runners)
    kv.Value.Update(time, time, time);

The only place runners is mutated is runners.Add(...) in GetOrCreateRunner, which runs lazily the first time a given (TValue, TOptions, TAdapter) motion type is scheduled on the dispatcher.

During runner.Update(...), motion callbacks fire (per-frame Bind actions, WithOnComplete, WithOnCancel, loop callbacks). If such a callback schedules the first motion of a not-yet-seen type through this dispatcher, GetOrCreateRunner adds a new entry to runners while the foreach is iterating it, throwing:

InvalidOperationException: Collection was modified; enumeration operation may not execute.
  System.Collections.Generic.Dictionary`2+Enumerator[TKey,TValue].MoveNext ()
  LitMotion.ManualMotionDispatcher.Update (System.Double deltaTime) at ManualMotionDispatcher.cs:82

It's intermittent because it only triggers the first time each new motion type is introduced from inside a callback during Update. Reset() has the same latent issue (a cancel callback can schedule a new-type motion).

Fix

Iterate a snapshot of runners.Values instead of the live dictionary, so a runner added by a callback mid-iteration no longer invalidates the enumeration.

The snapshot uses a reused List<IUpdateRunner> buffer, filled via AddRange(runners.Values) (which takes List's ICollection.CopyTo fast path; Dictionary caches its ValueCollection). So it stays allocation-free after warmup — no per-frame allocation is introduced. A runner created during Update simply ticks on the next frame.

Notes

• No public API change.
• Applied to both Update and Reset.