feat(verifier): name extractors for diff surfacing

bindExtract accepts an optional name argument; falls back to
extractor_N when omitted. The name is stored on extractorState
alongside prev/curr value caches that the next change will use to
emit per-step diffs.

internal/verifier/bindings.go

@@ -10,6 +10,12 @@ import (
 type extractorState struct {
 	getter goja.Callable
 	handle *goja.Object
+	name   string
+	// prev/curr cache the JSON-encoded extractor values from the prior and
+	// current PushSnapshot, used by ChangedExtractors to surface per-step
+	// diffs in the trace.
+	prev []byte
+	curr []byte
 }
 
 type formulaState struct {
@@ -139,19 +145,29 @@ func (v *Verifier) installRuntimeBindings() error {
 }
 
 func (v *Verifier) bindExtract(call goja.FunctionCall) goja.Value {
-	if len(call.Arguments) != 1 {
-		panic(v.runtime.NewTypeError("extract requires exactly one argument"))
+	if len(call.Arguments) < 1 || len(call.Arguments) > 2 {
+		panic(v.runtime.NewTypeError("extract requires (getter) or (getter, name)"))
 	}
 	getter, ok := goja.AssertFunction(call.Arguments[0])
 	if !ok {
 		panic(v.runtime.NewTypeError("extract argument must be a function"))
 	}
+	name := ""
+	if len(call.Arguments) == 2 {
+		arg := call.Arguments[1]
+		if !goja.IsUndefined(arg) && !goja.IsNull(arg) {
+			name = arg.String()
+		}
+	}
+	if name == "" {
+		name = fmt.Sprintf("extractor_%d", len(v.extractors))
+	}
 
 	handle := v.runtime.NewObject()
 	_ = handle.Set("current", goja.Undefined())
 	_ = handle.Set("previous", goja.Undefined())
 
-	v.extractors = append(v.extractors, &extractorState{getter: getter, handle: handle})
+	v.extractors = append(v.extractors, &extractorState{getter: getter, handle: handle, name: name})
 	return handle
 }
 

internal/verifier/verifier_test.go

@@ -894,6 +894,26 @@ func TestSelectorStringFromJS_CanonicalGrammar(t *testing.T) {
 	}
 }
 
+// TestExtract_DefaultsAndNamedNames verifies bindExtract assigns a fallback
+// `extractor_N` name when no name is supplied and respects an explicit one.
+func TestExtract_DefaultsAndNamedNames(t *testing.T) {
+	verifier := newVerifier(t)
+	mustLoad(t, verifier, `
+		__sanderling__.extract(state => 1);
+		__sanderling__.extract(state => 2, "ledgerRows");
+		__sanderling__.extract(state => 3);
+	`)
+	if len(verifier.extractors) != 3 {
+		t.Fatalf("extractors registered: got %d, want 3", len(verifier.extractors))
+	}
+	want := []string{"extractor_0", "ledgerRows", "extractor_2"}
+	for i, name := range want {
+		if got := verifier.extractors[i].name; got != name {
+			t.Errorf("extractor %d name: got %q, want %q", i, got, name)
+		}
+	}
+}
+
 // TestSelectorStringFromJS_NullEmpty verifies that nil/undefined args produce
 // an empty string instead of "null"/"undefined" garbage.
 func TestSelectorStringFromJS_NullEmpty(t *testing.T) {