Complete start_now with no senders

[fallintoplace]

Summary

• mark empty start_now storage as already complete
• add a regression test for start_now(scope)

Why

start_now accepts an empty sender pack. The storage starts with a pending count of zero, but its joiner still points to the waiting state. async_wait() can then install a waiter even though no child can complete the storage. Initializing the joiner to the existing completed marker makes an empty pack complete immediately.

Testing

• cmake --build build --target test.exec -j 8
• ./build/test/exec/test.exec '[async_scope][start_now]'

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.