Return the Promises waiter count for inspection

[OskarEichler]

Returns the AtomicFixnum value used by the lock-free event waiter registry. The current inspection path calls each on AtomicFixnum and raises instead of reporting the count.

Verified by focused Promises specs and models; the review composite passes 2,796 examples with no failures.

[bensheldon]

Please add a test for the behavior as it sounds like this has existed because it's untested.

[OskarEichler]

Added a regression spec that checks the waiter count before, during, and after a blocked wait. The focused Promises spec passes: 61 examples, 0 failures.

[OskarEichler]

Follow-up: the first regression spec used an ensure form that Ruby 2.3 and 2.4 could not parse, which caused the three legacy-Ruby CI failures. I rewrote the cleanup using an explicit begin/ensure, removed the unnecessary safe-navigation calls, and added the missing assertion that the waiter count returns to zero after resolution.

The focused Promises spec now passes with 61 examples and 0 failures on Ruby 4.0.6, and the updated spec is syntactically compatible with the legacy parser form exercised by CI.