feat(locator): form actions: clear, check, uncheck, isChecked, selectOption, inputValue #133

Form controls are only reachable through BasePage helpers (dropdowns) or the raw WebElement. LocatorAssert already has isChecked() but Locator has no matching check() / uncheck() / isChecked(). Add to Locator: clear(), check(), uncheck(), isChecked(), selectOption(String text), selectOptionByValue(String), inputValue(). check()/uncheck() should be idempotent (no click if already in the requested state).

Acceptance: tests pass (unit tests under src/test, plus a case against a real page in the sample project if it needs a browser); Javadoc on each public method; a changelog entry in docs-site/docs/changelog.md; one line in the locators guide. Terminal actions must follow the existing contract: resolve from scratch on every poll and retry stale/not-yet-present elements until timeouts.explicit (see #67/#68). See CONTRIBUTING.md; comment to claim it.