Add completeExceptionally, cancelAndJoin, and refine LaunchPool spec

- CompletableDeferred.completeExceptionally(exception): fails the deferred
  with a Lyng exception; the original exception object is preserved as
  errorObject so assertThrows/catch see the correct class (works for both
  built-in ObjException and user-declared class instances)
- LaunchPool: add maxQueueSize parameter (suspends launch when full),
  add cancelAndJoin() for graceful shutdown that waits for running tasks,
  clarify all semantics in the extern declaration comments
- Two tests for completeExceptionally (built-in and user-defined exception)
- Fix parallelism.md channel doc-test to be illustration-only (avoid
  flakiness from coroutine scheduling in the doc-test runner)

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

lynglib/src/commonTest/kotlin/CoroutinesTest.kt

@@ -58,6 +58,38 @@ class TestCoroutines {
         )
     }
 
+    @Test
+    fun testCompletableExceptionally() = runTest {
+        eval(
+            """
+            val done = CompletableDeferred()
+
+            launch {
+                delay(10)
+                done.completeExceptionally(IllegalStateException("boom"))
+            }
+
+            assert(!done.isCompleted)
+            assertThrows(IllegalStateException) { done.await() }
+            assert(done.isCompleted)
+        """.trimIndent()
+        )
+    }
+
+    @Test
+    fun testCompletableExceptionallyWithCustomException() = runTest {
+        eval(
+            """
+            class MyError(msg) : Exception(msg) {}
+
+            val done = CompletableDeferred()
+            done.completeExceptionally(MyError("custom failure"))
+
+            assertThrows(MyError) { done.await() }
+        """.trimIndent()
+        )
+    }
+
     @Test
     fun testDeferredCancel() = runTest {
         eval(