fix(screenDom): prioritize known app roots and skip empty containers

[kevinccbsg]

Summary

• Make screenDom query scoping robust against injected body siblings (sandbox overlays, browser extensions) that currently silently hijack queries by landing before the app root in DOM order.
• Resolve the container in priority order: user-configured rootSelector → #root / #app / app-root → first non-empty direct body child → document.body.
• Add rootSelector?: string option on InitTWDOptions, wired through initTWD to a package-internal setter (no new public exports from twd-js).
• Emit a one-time-per-session console.warn when the heuristic fallback fires without a configured rootSelector, pointing users to the new option.

Motivation

Reproduced on CodeSandbox Devbox: CodeSandbox injects <div id=\"highlighter\"> before <div id=\"root\"> for its click-to-inspect overlay, and every screenDom.getByText(...) silently fails with "element not found" because the (empty) highlighter became the scoped container. Any browser extension or framework overlay that injects a body sibling has the same effect. First-install experience on a sandbox is a silent breakage.

What changes

• src/proxies/screenDom.ts — priority list + skip-empty heuristic + one-time warn + module-state reset for tests.
• src/bundled.tsx — rootSelector?: string on InitTWDOptions; initTWD forwards to the internal setter before initTests / initRequestMocking.
• Docs: docs/testing-library.md (rewritten "How screenDom Works" + troubleshooting); docs/getting-started.md (new commented option in the initTWD example).
• CHANGELOG: additional bullet under the existing unreleased 1.7.2 entry — no version bump.

Non-goals

• No public re-export of setRootSelector or resetScreenDomState — those are package-internal.
• No signature change to initTests (React-path power users who need a custom root use initTWD).
• No change to screenDomGlobal.

Test plan

• npm run test:ci — 391/391 passing across 54 files
• npm run docs:build — VitePress build clean
• npm run build — succeeds; the TS errors in src/plugin/* are pre-existing and unrelated
• New unit tests cover: priority order (#root > decoy, #app fallback, app-root fallback), skip-empty heuristic, configured selector wins, configured-but-missing falls through, warn fires exactly once per session
• Manual verification on CodeSandbox (run an example app and confirm the #highlighter regression no longer manifests)

🤖 Generated with Claude Code