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/archive/archive_test.go

@@ -0,0 +1,37 @@
+package archive
+
+import (
+	"testing"
+	"time"
+)
+
+func TestConfig_Validate_Defaults(t *testing.T) {
+	c := Config{}
+	c = c.validate()
+
+	if c.Endpoint != "https://archive.ph" {
+		t.Errorf("Endpoint = %q, want %q", c.Endpoint, "https://archive.ph")
+	}
+	if c.Timeout == nil {
+		t.Fatal("Timeout should not be nil after validate")
+	}
+	if *c.Timeout != 1*time.Hour {
+		t.Errorf("Timeout = %v, want %v", *c.Timeout, 1*time.Hour)
+	}
+}
+
+func TestConfig_Validate_Preserves(t *testing.T) {
+	timeout := 5 * time.Minute
+	c := Config{
+		Endpoint: "https://archive.org",
+		Timeout:  &timeout,
+	}
+	c = c.validate()
+
+	if c.Endpoint != "https://archive.org" {
+		t.Errorf("Endpoint = %q, want %q", c.Endpoint, "https://archive.org")
+	}
+	if *c.Timeout != 5*time.Minute {
+		t.Errorf("Timeout = %v, want %v", *c.Timeout, 5*time.Minute)
+	}
+}