test(word-addin): run the e2e suite under WebKit and gate it in CI

WHY THIS MATTERS
The task pane's headline fix defends the transcript's scroll position
against WKWebView, but the assertions that prove it only fail under
WebKit. WebKit's scroll anchoring ignores `overflow-anchor: none` and
rewrites scrollTop whenever a descendant (e.g. the collapsing
"Working -> Completed in N steps" strip) resizes; Chromium honours the
opt-out. A chromium-only suite therefore stays green even with the fix
fully reverted -- the tests pass vacuously.

WHAT IS A VACUOUS TEST
A test that asserts a property the environment can never violate. It
looks like coverage but can't fail, so regressions ship silently. The
cure is to run the assertion in the environment where the property is
actually at risk -- here, a WebKit browser, the same engine Word on
macOS embeds.

HOW IT WORKS
- playwright.config.ts gains a `webkit` project (Desktop Safari), so
  `npm run test:e2e` runs every spec in both engines by default; a
  `test:e2e:webkit` script exists for targeted debugging.
- The orphaned playwright.webkit.temp.config.ts (referenced by no
  script, doc, or CI, and invisible to every tsconfig) is deleted --
  its only non-duplicated content was the webkit device entry.
- A new path-filtered .github/workflows/word-addin.yml gates
  typecheck + the two-browser suite on every change under word-addin/.
  The webServer timeout rises 180s->300s because in CI that command
  performs a cold typecheck + production webpack build, and a webServer
  timeout aborts the run un-retried.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015U1iPxsADQ2yuksNmDuvvW