controllogic: fix data race in setPath nested-array assignment

setPath mutated nested sub-arrays in place; since graph-local slices (and
their nested sub-slices) may be read concurrently by other flow goroutines,
an action.array.set with a multi-level path raced readers of the same local.
The single-threaded TS source mutates in place safely, but the Go port runs
flows on concurrent goroutines, so setPath now copies on descent (every level
returns a freshly allocated slice). Adds a -race regression test that
concurrently drives nested set + inner-element reads, plus a nested-set
correctness test asserting the prior stored value is not mutated.

Found by whole-branch review (Opus); missed by the per-task reviews.

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

internal/controllogic/engine_test.go

@@ -3,6 +3,7 @@ package controllogic
 import (
 	"context"
 	"reflect"
+	"sync"
 	"testing"
 )
 
@@ -108,6 +109,79 @@ func TestArrayPopNode(t *testing.T) {
 	}
 }
 
+// TestArraySetNodeNested verifies set with a comma-separated path mutates a nested
+// sub-array, and (crucially) that setPath does not mutate the previously stored slice
+// in place — the stored value must be replaced by a fresh tree.
+func TestArraySetNodeNested(t *testing.T) {
+	g := Graph{
+		ID: "g", Name: "n",
+		StateVars: []StateVar{{Name: "grid", Type: "array", Initial: "[[1,2],[3,4]]", Sizing: "dynamic"}},
+		Nodes: []Node{
+			{ID: "t", Kind: "trigger.timer", Params: map[string]string{"interval": "1000"}},
+			{ID: "s", Kind: "action.array.set", Params: map[string]string{"array": "grid", "index": "0,1", "expr": "9"}},
+		},
+		Wires: []Wire{{From: "t", To: "s"}},
+	}
+	cg := compile(g)
+	before := cg.getLocal("grid")
+	cg.engine = &Engine{}
+	cg.genCtx = context.Background()
+	R := func(ds, name string) Value {
+		if ds == "local" {
+			return cg.getLocal(name)
+		}
+		return 0.0
+	}
+	cg.follow("t", "out", &runCtx{fired: "t", resolve: R})
+	want := []Value{[]Value{1.0, 9.0}, []Value{3.0, 4.0}}
+	if got := cg.getLocal("grid"); !reflect.DeepEqual(got, want) {
+		t.Fatalf("after nested set grid = %#v", got)
+	}
+	// The pre-mutation snapshot must be untouched (no shared-backing in-place write).
+	if !reflect.DeepEqual(before, []Value{[]Value{1.0, 2.0}, []Value{3.0, 4.0}}) {
+		t.Fatalf("setPath mutated the prior stored value in place: %#v", before)
+	}
+}
+
+// TestArraySetNodeConcurrentNoRace drives many concurrent set+read flows against the
+// same nested-array local; with -race it guards the setPath copy-on-descend fix.
+func TestArraySetNodeConcurrentNoRace(t *testing.T) {
+	g := Graph{
+		ID: "g", Name: "n",
+		StateVars: []StateVar{{Name: "grid", Type: "array", Initial: "[[0,0],[0,0]]", Sizing: "dynamic"}},
+		Nodes: []Node{
+			{ID: "t", Kind: "trigger.timer", Params: map[string]string{"interval": "1000"}},
+			{ID: "s", Kind: "action.array.set", Params: map[string]string{"array": "grid", "index": "0,0", "expr": "1"}},
+		},
+		Wires: []Wire{{From: "t", To: "s"}},
+	}
+	cg := compile(g)
+	cg.engine = &Engine{}
+	cg.genCtx = context.Background()
+	R := func(ds, name string) Value {
+		if ds == "local" {
+			return cg.getLocal(name)
+		}
+		return 0.0
+	}
+	var wg sync.WaitGroup
+	for i := 0; i < 32; i++ {
+		wg.Add(2)
+		go func() { defer wg.Done(); cg.follow("t", "out", &runCtx{fired: "t", resolve: R}) }()
+		go func() {
+			defer wg.Done()
+			// Read the inner element concurrently; with the in-place setPath bug the
+			// writer mutates this same sub-array backing, producing a data race.
+			if arr, ok := cg.getLocal("grid").([]Value); ok && len(arr) > 0 {
+				if sub, ok := arr[0].([]Value); ok && len(sub) > 0 {
+					_ = sub[0]
+				}
+			}
+		}()
+	}
+	wg.Wait()
+}
+
 func TestCompileInitsLocalsFromDecls(t *testing.T) {
 	g := Graph{
 		ID: "g", Name: "n",