4.x: Streamable + timer + interval; cleanups

[akarnokd]

Yep, hand crafting state machines of CompletionStages is tedious. Structured concurrency and its join might have helped here, but that requires using a virtual thread at every junktion.

Just like there is non-blocking concurrency, we can do ˙whenComplete style free threaded join patterns, aka non-executor concurrency.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.637 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/main/java/io/reactivex/rxjava4/core/CompletionStageDisposable.java: 4.620
src/main/java/io/reactivex/rxjava4/core/Streamable.java: 4.608
src/main/java/io/reactivex/rxjava4/core/Streamer.java: 4.673
src/main/java/io/reactivex/rxjava4/core/config/StreamableInterceptConfig.java: 4.911
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableEmpty.java: 4.886
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableError.java: 4.808
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFilter.java: 4.271
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFlatMap.java: 4.173
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromArray.java: 4.640
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromIterable.java: 4.628
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromPublisher.java: 4.561
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableHide.java: 4.851
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableIntercept.java: 4.628
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableJust.java: 4.682
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableMap.java: 4.692
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableMapOptional.java: 4.335
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableNever.java: 4.840
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableRange.java: 4.635
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableRangeLong.java: 4.660
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFlatMapTest.java: 4.776

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.70%. Comparing base (fc87d8c) to head (86c494f).

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8213      +/-   ##
============================================
- Coverage     98.71%   98.70%   -0.01%     
- Complexity     6902     6925      +23     
============================================
  Files           791      793       +2     
  Lines         48889    49022     +133     
  Branches       6605     6628      +23     
============================================
+ Hits          48260    48389     +129     
- Misses          446      448       +2     
- Partials        183      185       +2     

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