refactor(artwork): make the trace nil-safe in one place only

add already handles a nil trace, so record's own guard was dead; Steps was the
odd one out and would panic where every other method tolerates nil.

core/artwork/resolve.go

@@ -56,9 +56,7 @@ func (c *chainState) try(candidate string, res resolution, ok bool) (resolution,
 }
 
 func (c *chainState) record(candidate string, out Outcome, detail string) {
-	if c.trace != nil {
-		c.trace.add(traceStep{Candidate: candidate, Outcome: out, Detail: detail})
-	}
+	c.trace.add(traceStep{Candidate: candidate, Outcome: out, Detail: detail})
 }
 
 // exhausted is the outcome when no source in the chain yielded an image.

core/artwork/trace.go

@@ -52,6 +52,9 @@ func (t *chainTrace) add(step traceStep) {
 }
 
 func (t *chainTrace) Steps() []traceStep {
+	if t == nil {
+		return nil
+	}
 	t.mu.Lock()
 	defer t.mu.Unlock()
 	return slices.Clone(t.steps)

core/artwork/trace_test.go

@@ -56,6 +56,7 @@ var _ = Describe("chainTrace", func() {
 	It("does not panic when the trace is nil", func() {
 		var t *chainTrace
 		Expect(func() { t.add(traceStep{Candidate: "cover.*", Outcome: OutcomeMiss}) }).ToNot(Panic())
+		Expect(t.Steps()).To(BeEmpty(), "a nil trace collects nothing, so reading it must be as safe as writing it")
 	})
 
 	It("is safe to use concurrently", func() {