feat: harden support, localize knowledge and add operator availability [skip ci]

Source-only publication of reviewed commit 64e96a7f423673dc29ad56485be9fc79bce8af1c.

Passed in isolation: focused frontend/backend tests, race checks, TypeScript typecheck, lint, Go compilation, Flowgram build, and deterministic SDK build/tests.

Not accepted as a production release: Next.js export/native assembly remains blocked by environment memory limits; browser interaction and MySQL staging acceptance remain unverified. No production server, database/schema, runtime configuration, deployment, release tag, or workflow execution was performed.

Expected source tree: 0a4a7528e9.

internal/pkg/i18nx/context_test.go

@@ -0,0 +1,45 @@
+package i18nx
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestWithLocalePreservesContext(t *testing.T) {
+	type testKey struct{}
+	deadline := time.Now().Add(time.Minute)
+	parent, cancel := context.WithDeadline(context.WithValue(context.Background(), testKey{}, "request-value"), deadline)
+	defer cancel()
+	localized := WithLocale(parent, "ru")
+	if got := LocaleFromContext(localized); got != LocaleRuRU {
+		t.Fatalf("locale = %q", got)
+	}
+	if got := localized.Value(testKey{}); got != "request-value" {
+		t.Fatalf("parent value lost: %v", got)
+	}
+	if got, ok := localized.Deadline(); !ok || !got.Equal(deadline) {
+		t.Fatalf("deadline lost: %v, %v", got, ok)
+	}
+	cancel()
+	if !errors.Is(localized.Err(), context.Canceled) {
+		t.Fatalf("cancellation lost: %v", localized.Err())
+	}
+}
+
+func TestLocaleFromContextDefaultsAndIsolation(t *testing.T) {
+	for _, ctx := range []context.Context{nil, context.Background(), context.WithValue(context.Background(), "i18nx.locale", LocaleZhCN)} {
+		if got := LocaleFromContext(ctx); got != DefaultLocale {
+			t.Fatalf("missing explicit locale = %q, want default %q", got, DefaultLocale)
+		}
+	}
+	english := WithLocale(context.Background(), LocaleEnUS)
+	russian := WithLocale(english, LocaleRuRU)
+	if LocaleFromContext(english) != LocaleEnUS || LocaleFromContext(russian) != LocaleRuRU {
+		t.Fatal("child locale changed parent locale")
+	}
+	if got := LocaleFromContext(WithLocale(nil, "unsupported")); got != DefaultLocale {
+		t.Fatalf("invalid locale = %q", got)
+	}
+}