Slate Browser E2E Surface Widening

Goal

Bring read-only and shadow-dom into the dedicated current-example browser lane instead of only proving them through the replacement matrix.

Result

• added dedicated browser tests for:
  • read-only
  • shadow-dom
  • iframe
  • plaintext
• widened test:slate-browser:e2e and test:slate-browser:e2e:local to carry those examples and the broader current example suite
• kept the proof-lane matrix aligned with the widened current-example e2e lane

Verification

• bash ./scripts/run-slate-browser-local.sh 3100 /examples/read-only "yarn build:slate-browser:playwright && yarn exec playwright test playwright/integration/examples/read-only.test.ts playwright/integration/examples/shadow-dom.test.ts --project=chromium --workers=1"
• bash ./scripts/run-slate-browser-local.sh 3100 /examples/plaintext "yarn build:slate-browser:playwright && yarn exec playwright test playwright/integration/examples/iframe.test.ts playwright/integration/examples/plaintext.test.ts --project=chromium --workers=1"
• yarn test:slate-browser:e2e:local