feat(locator): waitFor, allTextContents, element screenshot #135

Add to Locator: waitFor(State) (visible/hidden/attached/detached, retrying until timeouts.explicit), allTextContents() (all matches' text, no wait on empty), and screenshot() returning the element's PNG bytes (and a variant that writes to a path). Useful for list assertions and for attaching element shots to the trace viewer.

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.