Add Channel primitive for coroutine inter-task communication

Exposes Kotlin's Channel<Obj> to Lyng scripts as a first-class type
with rendezvous, buffered, and unlimited capacity modes. Supports
suspending send/receive, non-suspending tryReceive, close/drain
semantics, and isClosedForSend/isClosedForReceive properties.

Also fixes a pre-existing typo in BookTest.kt that blocked JVM test
compilation, and adds Channel reference docs and a parallelism.md section.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lynglib/src/jvmTest/kotlin/BookTest.kt

@@ -236,7 +236,7 @@ suspend fun runDocTests(fileName: String, bookMode: Boolean = false) {
     val bookScope = Scope()
     var count = 0
     parseDocTests(fileName, bookMode).collect { dt ->
-        if (bookMode)imp dt.test(bookScope)
+        if (bookMode) dt.test(bookScope)
         else dt.test()
         count++
     }