Preserve completion scheduler in timed fallbacks

[fallintoplace]

Summary

The timed scheduler fallbacks currently use let_value(just(), ...) to keep now() lazy. let_value does not expose a completion scheduler, so both fallback senders lose get_completion_scheduler<set_value_t> even when the delegated timed sender advertises it.

This adds a small internal wrapper that forwards the fallback sender’s existing completion behavior and signatures and adds the set-value completion scheduler. Tests cover both fallback directions.

Testing

• build/test/exec/test.exec
• build/test/exec/test.exec "[timed_scheduler]"
• build/test/exec/test.exec "[timed_thread_scheduler]"

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[ericniebler]

should this be in namespace experimental::execution instead of STDEXEC?

[ericniebler]

/ok to test 10db6f0

[ericniebler]

/ok to test 70dddb5

[ericniebler]

i'm ambivalent about merging this since it is a work-around for a problem in the let_ senders that #2077 aims to fix.