refactor: restructure API, deduplicate code, expand test coverage

- Extract shared DeferClose helper, removing 14 duplicate copies
- Rename PlayWright-prefixed types to cleaner names (BrowserOptions,
  BrowserSelection, NewBrowser, etc.)
- Rename fields: ServerAddress, RequireServer (was DontLaunchOnConnectFailure)
- Extract shared initBrowser/mergeOptions into browser_init.go,
  deduplicating ~120 lines between NewBrowser and NewInteractiveBrowser
- Remove unused locator field from document struct
- Add tests for all previously untested packages (archive, aislegopher,
  wegmans, useragents, powerball) and expand existing test suites
- Add MIGRATION.md documenting all breaking API changes

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

sites/duckduckgo/duckduckgo_test.go

@@ -83,3 +83,34 @@ func TestConfig_ToSearchURL_NoRegion(t *testing.T) {
 		t.Errorf("kl should be empty when no region, got %q", u.Query().Get("kl"))
 	}
 }
+
+func TestConfig_ToSearchURL_Scheme(t *testing.T) {
+	c := Config{SafeSearch: SafeSearchOff}
+	u := c.ToSearchURL("test")
+
+	if u.Scheme != "https" {
+		t.Errorf("Scheme = %q, want %q", u.Scheme, "https")
+	}
+}
+
+func TestConfig_ToSearchURL_SpecialChars(t *testing.T) {
+	c := Config{SafeSearch: SafeSearchOff}
+	u := c.ToSearchURL("go lang & testing")
+
+	if u.Query().Get("q") != "go lang & testing" {
+		t.Errorf("q = %q, want %q", u.Query().Get("q"), "go lang & testing")
+	}
+}
+
+func TestResult_ZeroValue(t *testing.T) {
+	var r Result
+	if r.URL != "" || r.Title != "" || r.Description != "" {
+		t.Error("zero-value Result should have empty fields")
+	}
+}
+
+func TestDefaultConfig_SafeSearch(t *testing.T) {
+	if DefaultConfig.SafeSearch != SafeSearchOff {
+		t.Errorf("DefaultConfig.SafeSearch = %d, want %d", DefaultConfig.SafeSearch, SafeSearchOff)
+	}
+}