4.x: Streamable mapOptional +perf

[akarnokd]

Uses the same Claude Fable state machine approach as filter, plus the support for EnumerableSource and DeferredEnumerableSource-based flows.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.573 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/jmh/java/io/reactivex/rxjava4/streamable/StreamableMapOptionalPerf.java: 4.814
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableHelper.java: 4.426
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableMapOptional.java: 4.252
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableMapOptionalTest.java: 4.801

✅ No secrets or suspicious high-entropy strings found.

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

[codecov]

Codecov Report

❌ Patch coverage is 96.62921% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.59%. Comparing base (258caeb) to head (9e71899).

Files with missing lines	Patch %	Lines
...al/operators/streamable/StreamableMapOptional.java	96.42%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8240      +/-   ##
============================================
- Coverage     98.62%   98.59%   -0.04%     
- Complexity     7154     7157       +3     
============================================
  Files           822      822              
  Lines         50625    50683      +58     
  Branches       6879     6889      +10     
============================================
+ Hits          49930    49971      +41     
- Misses          475      483       +8     
- Partials        220      229       +9     

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