docs: refresh README and add repo social preview

[pietechsolution]

Documentation and repository metadata pass for the React Native SDK.

README

• Restructured to match the Flutter and iOS package READMEs: install, quick
  start, runnable example, security boundary, runtime/WebView architecture,
  commands, events, layout and lifecycle, FAQ, ecosystem table, development.
• Added a TypeScript badge and stated explicitly that declaration files are
  published with the package (dist/index.d.ts / dist/index.d.cts).
• Documented the peer dependency floors (react >= 18.2.0,
  react-native >= 0.72.0, react-native-webview >= 13.0.0) from
  package.json.
• Added a section describing how to run example/, including the
  EXPO_PUBLIC_SEATLAYER_EVENT variable the example actually reads.
• Added a private-inventory buyerAccessTokenProvider snippet matching
  SeatLayerConfiguration in src/types.ts.
• Added a FAQ covering setup, the native-component-vs-WebView question, Expo,
  temporary holds, payment providers, and supported versions.
• Added the SeatLayer SDK ecosystem table used across the sibling repos, with
  this package marked.

Every factual claim was checked against the repo source: the command list
against src/controller.ts, the event list against SeatLayerEventMap in
src/types.ts, the hosted runtime pin seatlayer-js@0.66.0 and origin against
src/types.ts, the 15s command deadline and per-event-name sequence filtering
against src/bridge/client.ts, declaration output against tsup.config.ts and
the exports map, and the absence of any custom native module by searching the
tree for podspec/Java/Kotlin/Swift/Objective-C sources. All external links were
checked for a 200 response.

Repository metadata

• Adds .github/social-preview.png (1280x640). The image still has to be
  uploaded by hand in Settings → Social preview; the API cannot set it.
• The About description and topics are being updated separately via the API.

No source, build, or published-package changes. Not for merge until reviewed.