Modernize and cleanup: Swift 5.9, concurrency safety, and documentation

[maximkhatskevich]

Summary

Brings the codebase to Swift 5.9 standard with Swift 6.0 readiness.

Changes

Important

• Bump swift-tools-version to 5.9, macOS minimum to 12+
• Fix data race in waitForFirstResult() — added .receive(on: DispatchQueue.main) serialization + resumed guard
• Switch ensureMainThread() to DispatchQueue.main for consistency
• Apply @Sendable consistently across all operators and Pipeline static methods
• Add tests for waitForFirstResult() — success, failure, and empty completion cases

Enhancements

• Add LocalizedError conformance to FailedConditionCheck and CompletedWithoutValue
• Add async variants to SimpleWrapper — map, inspect, mutate
• Make FailedConditionCheck public
• Add CompletedWithoutValue error type for publishers that complete without a value
• Remove unimplemented operators (.!/, ./>, !!)

Cleanup

• Fix typos in comments
• Remove //infix comment noise from all operator functions
• Remove dead //import XCERequirement from test files
• Standardize doc comments to /// style
• Update README with current badges, features, and documentation

Test Results

45/45 tests pass (up from 42 — 3 new waitForFirstResult tests).

Files Changed

• Package.swift — version + platform bumps
• Sources/Core/Extensions/Publisher+Helpers.swift — concurrency fix
• Sources/Core/Operators.swift — cleanup + @Sendable
• Sources/Core/Pipeline.swift — public types + LocalizedError + @Sendable
• Sources/Core/SimpleWrapper.swift — async variants + @Sendable
• Sources/Core/Take.swift — typo fix
• Tests/AllTests/PublisherTests.swift — new tests
• Tests/AllTests/OperatorsTests.swift — dead import removed
• Tests/AllTests/OperatorsAsyncTests.swift — dead import removed
• README.md — full update

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Pull Request opened by Augment Code with guidance from the PR author