P4123: refresh once `std::execution::task` ships

[MungoG]

Vinnie (DM, 2026-05-28 18:12):

P4123 will need to be updated when task ships.

Action:

1. Hold the issue open with no edit until std::execution::task
   lands in its final form (likely a published WG21 revision or
   a merged paper that nails its surface).
2. When that happens, revisit any places in P4123 that compare
   our model against task or that hedge against task's final
   shape, and tighten the wording.
3. Close once P4123 has been re-revved to reflect the shipped
   form of task.