Add Linux support

[alexey1312]

Description

Add Linux support for the swift-yyjson library with platform-specific workarounds for swift-corelibs-foundation limitations.

Changes

• Add Linux CI workflow with Swift 6.1 testing
• Add Linux traits CI job for noReader and noWriter traits
• Implement platform-specific isBoolNumber() using NSNumber singleton comparison on Linux
• Implement NSMutableString workaround using mutableCopy() on Linux
• Replace preconditionFailure with throwing YYJSONError for better error handling
• Add documentation for Linux-specific workarounds
• Skip readWithMutableLeaves test on Linux (swift-corelibs-foundation limitation)
• Update actions/checkout to v6

Testing

• All 465 tests pass on macOS
• CI configured to run on Linux with Swift 6.1

[alexey1312]

@mattt it is the last one for today, please look when you have time)

[Copilot]

Pull request overview

This pull request adds Linux support to the swift-yyjson library, including platform-specific workarounds for swift-corelibs-foundation limitations and comprehensive Linux CI testing with Swift 6.1.

Changes:

• Added Linux CI workflows with Swift 6.1 testing for both standard and trait configurations
• Implemented platform-specific boolean NSNumber detection using singleton comparison on Linux
• Implemented NSMutableString creation workaround using mutableCopy() on Linux
• Conditionally skipped readWithMutableLeaves test on Linux due to NSDictionary type casting limitation
• Updated GitHub Actions checkout to v6

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
.github/workflows/ci.yml	Added two new Linux CI jobs: test-linux for standard builds and test-linux-traits for trait-specific testing; updated all checkout actions to v6
Sources/YYJSON/Serialization.swift	Added isBoolNumber() helper with Darwin/Linux implementations; added platform-specific NSMutableString creation using mutableCopy() on Linux
Tests/YYJSONTests/SerializationTests.swift	Made readWithMutableLeaves test Darwin-only with explanatory comment about Linux NSDictionary behavior

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[mattt]

Looking good! Thanks for all of your help, @alexey1312 🫶

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

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