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>

document.go

@@ -25,7 +25,6 @@ type document struct {
 	pw      *playwright.Playwright
 	browser playwright.Browser
 	page    playwright.Page
-	locator playwright.Locator
 }
 
 func newDocument(pw *playwright.Playwright, browser playwright.Browser, page playwright.Page) (Document, error) {