feat(locator): first() and last() #132

Locator has nth(int) but no first() / last(). They are the most common narrowing calls (getByRole(Role.LISTITEM).first()). Add both as thin, readable wrappers (last() must resolve at action time, not at build time).

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.