feat(keys): add --keys flag to dump for cross-host decryption

Consumer side of the cross-host key workflow (pairs with #599).
ApplyDump wires StaticProviders from a dump.json into matching
browsers, so dump --keys f.json -p /copied/data decrypts without
native retrievers.

crypto/keyretriever/dump_test.go

@@ -0,0 +1,41 @@
+package keyretriever
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestReadJSON_RejectsUnknownVersion(t *testing.T) {
+	input := bytes.NewBufferString(`{"version":"99","created_at":"2026-05-16T00:00:00Z","host":{"os":"linux","arch":"amd64"},"vaults":[]}`)
+	_, err := ReadJSON(input)
+	if err == nil {
+		t.Fatal("ReadJSON should reject unknown version, got nil error")
+	}
+	if !strings.Contains(err.Error(), "unsupported dump version") {
+		t.Errorf("error should mention unsupported version, got: %v", err)
+	}
+}
+
+func TestReadJSON_RejectsMissingVersion(t *testing.T) {
+	input := bytes.NewBufferString(`{"created_at":"2026-05-16T00:00:00Z","host":{"os":"linux","arch":"amd64"},"vaults":[]}`)
+	_, err := ReadJSON(input)
+	if err == nil {
+		t.Fatal("ReadJSON should reject empty version, got nil error")
+	}
+}
+
+func TestReadJSON_AcceptsCurrentVersion(t *testing.T) {
+	d := NewDump()
+	var buf bytes.Buffer
+	if err := d.WriteJSON(&buf); err != nil {
+		t.Fatalf("WriteJSON: %v", err)
+	}
+	parsed, err := ReadJSON(&buf)
+	if err != nil {
+		t.Fatalf("ReadJSON: %v", err)
+	}
+	if parsed.Version != DumpVersion {
+		t.Errorf("Version = %q, want %q", parsed.Version, DumpVersion)
+	}
+}