ParallelLoop: make loop tasks cancellable to avoid hang on Teardown

SimpleParallelTask/LoopRangeTask never overrode Cancellable(), so a
ThreadManager::Teardown() racing with an in-flight parallel loop would
drop them without ever calling counter_->Count(), leaving anyone
blocked in WaitableCounter::Wait() stuck forever. Cancel() now counts
down the same as a normal completion.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L4QAoxV2KY7ek4PcZw3WvY

Common/Thread/ParallelLoop.cpp

@@ -23,6 +23,16 @@ public:
 		counter_->Count();
 	}
 
+	// Cancellable so a Teardown() racing with an in-flight parallel loop still
+	// counts down the waiter's counter instead of leaving it blocked forever.
+	bool Cancellable() const override {
+		return true;
+	}
+
+	void Cancel() override {
+		counter_->Count();
+	}
+
 	std::function<void(int, int)> loop_;
 	WaitableCounter *counter_;