[WIP] Add multiplatform support for kotest variant

[Copilot]

• Make base module multiplatform (JVM + JS/Node.js)
  • Add okio as the cross-platform path/filesystem library
  • Move DeletionMode.kt, TestFiles.kt to commonMain (change java.nio.file.Path to okio.Path)
  • Rewrite DefaultTestFiles.kt as common code using okio, with expect/actual for synchronized access and OS-specific filename chars
  • Add jsMain implementations: JsNodeFileSystem, absolutize, invalidFileNameCharacters, platformFileSystem, synchronizedAccess
  • Add jvmMain implementations: absolutize, platformFileSystem, synchronizedAccess
  • Update base/build.gradle.kts for multiplatform
  • Move JVM tests to jvmTest, update for okio.Path (add OkioPathAssertions.kt, fix imports)
• Make kotest module multiplatform (JVM + JS/Node.js)
  • Move KotestTestFilesAdapter.kt, testFiles.kt to commonMain
  • Add expect/actual for testScopeName() (uses qualifiedName on JVM, simpleName on JS)
  • Add JS integration test (jsTest/KotestTestFilesIntegrationSpec.kt)
  • Update kotest/build.gradle.kts for multiplatform
• Update root build.gradle.kts for multiplatform publishing support
• Fix 2 remaining JVM test failures:
  • retains existing scope directories when entering a scope - scope directory clearing logic
  • escapes '\' in a scope name if necessary - backslash path splitting on Linux
• Run and verify JS tests pass

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