4.x: Add task coordination primitives, implement a basic concat() operator

[akarnokd]

Building blocks for more advanced coordination-heavy operators.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.76 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

.github/workflows/scorecard.yml: 4.760

✅ 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 31.18280% with 64 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.76%. Comparing base (d9d985c) to head (0f76972).
⚠️ Report is 11 commits behind head on 4.x.

Files with missing lines	Patch %	Lines
.../rxjava4/internal/util/AwaitCoordinatorStatic.java	21.05%	44 Missing and 1 partial ⚠️
...ain/java/io/reactivex/rxjava4/core/Streamable.java	47.36%	10 Missing ⚠️
.../internal/disposables/ListCompositeDisposable.java	0.00%	4 Missing ⚠️
...ctivex/rxjava4/internal/util/AwaitCoordinator.java	25.00%	3 Missing ⚠️
...ctivex/rxjava4/core/CompletionStageDisposable.java	50.00%	1 Missing ⚠️
...tivex/rxjava4/disposables/DisposableContainer.java	0.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (31.18%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8110      +/-   ##
============================================
- Coverage     98.90%   98.76%   -0.15%     
- Complexity     6862     6872      +10     
============================================
  Files           767      769       +2     
  Lines         48586    48659      +73     
  Branches       6508     6516       +8     
============================================
+ Hits          48053    48057       +4     
- Misses          398      462      +64     
- Partials        135      140       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.