feat(env): add nestedUnderJQPath for extracting nested subtrees by jq-style paths

Introduce `nestedUnderJQPath`, enabling subtree extraction using jq-style path parsing. Add associated utility functions (`splitJQPath`, `peelRedundantPathLeaf`, `buildNestedTreeAtPath`) and test coverage. Extend `.env Codec` to support path-specific subtree merges. Update `WithFile` to accept an optional jqPath for targeted merges.

env/jqpath_test.go

@@ -0,0 +1,69 @@
+package env
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestNestedUnderJQPath(t *testing.T) {
+	nested := map[string]any{
+		"noise": map[string]any{"x": "ignore"},
+		"service": map[string]any{
+			"name": "my-service",
+			"database": map[string]any{
+				"url": "postgres://db",
+			},
+		},
+	}
+	got := nestedUnderJQPath(nested, ".service")
+	want := map[string]any{
+		"name": "my-service",
+		"database": map[string]any{
+			"url": "postgres://db",
+		},
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("got %#v want %#v", got, want)
+	}
+}
+
+func TestNestedUnderJQPath_EmptyPathNoop(t *testing.T) {
+	n := map[string]any{"a": "1"}
+	if got := nestedUnderJQPath(n, ""); !reflect.DeepEqual(got, n) {
+		t.Fatalf("%#v", got)
+	}
+	if got := nestedUnderJQPath(n, "."); !reflect.DeepEqual(got, n) {
+		t.Fatalf("%#v", got)
+	}
+}
+
+func TestNestedUnderJQPath_MissingPath(t *testing.T) {
+	got := nestedUnderJQPath(map[string]any{"other": "v"}, ".service")
+	if len(got) != 0 {
+		t.Fatalf("got %#v", got)
+	}
+}
+
+func TestNestedUnderJQPath_PeelsRedundantServiceWrapper(t *testing.T) {
+	nested := map[string]any{
+		"service": map[string]any{
+			"service": map[string]any{
+				"name": "inner",
+			},
+		},
+	}
+	got := nestedUnderJQPath(nested, ".service")
+	want := map[string]any{
+		"name": "inner",
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("got %#v want %#v", got, want)
+	}
+}
+
+func TestNestedUnderJQPath_ScalarAtServiceIgnored(t *testing.T) {
+	got := nestedUnderJQPath(map[string]any{"service": "x"}, ".service")
+	if len(got) != 0 {
+		t.Fatalf("got %#v", got)
+	}
+}