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/commonMain/kotlin/net/sergeych/lyng/obj/ObjCompletableDeferred.kt

@@ -18,6 +18,7 @@
 package net.sergeych.lyng.obj
 
 import kotlinx.coroutines.CompletableDeferred
+import net.sergeych.lyng.ExecutionError
 import net.sergeych.lyng.Scope
 import net.sergeych.lyng.miniast.ParamDoc
 import net.sergeych.lyng.miniast.addFnDoc
@@ -43,6 +44,30 @@ class ObjCompletableDeferred(val completableDeferred: CompletableDeferred<Obj>):
                 thisAs<ObjCompletableDeferred>().completableDeferred.complete(args.firstAndOnly())
                 ObjVoid
             }
+            addFnDoc(
+                name = "completeExceptionally",
+                doc = "Fail this deferred with the given exception. Awaiting it will then throw that exception. " +
+                    "Subsequent calls have no effect. The argument must be an `Exception` instance.",
+                params = listOf(ParamDoc("exception", type("lyng.Exception"))),
+                returns = type("lyng.Void"),
+                moduleName = "lyng.stdlib"
+            ) {
+                val ex = requiredArg<Obj>(0)
+                val scope = requireScope()
+                val msg = when (ex) {
+                    is ObjException -> ex.message.value
+                    else -> ex.toString(scope).value
+                }
+                val pos = when (ex) {
+                    is ObjException -> ex.scope.pos
+                    else -> scope.pos
+                }
+                // Always carry the original Lyng object as errorObject so that
+                // assertThrows / catch clauses see the correct exception class.
+                val cause = ExecutionError(ex, pos, msg)
+                thisAs<ObjCompletableDeferred>().completableDeferred.completeExceptionally(cause)
+                ObjVoid
+            }
         }
     }
 }