test(e2e/router): exercise both router strategies

Add a parameterised e2e suite that runs the same 22 scenarios against
both router implementations (Navigation API and legacy History API).
The legacy strategy is forced on via `window.__VP_USE_LEGACY_ROUTER__`,
set via Playwright `addInitScript` on a second browser context — the
shared browser and default `page` / `goto` from vitestSetup.ts are
reused for the Navigation API run, so we don't relaunch Playwright.

Scenarios covered, for each mode:

  - strategy selection reflects the flag
  - link click performs SPA navigation (no full reload)
  - hash anchor updates URL, scrolls into view, focuses target heading
  - back / forward navigate between entries
  - scroll position restored on back traversal
  - onBeforeRouteChange / onAfterRouteChange fire for full and hash navs
  - onBeforeRouteChange returning false cancels the nav
  - programmatic router.go
  - router.go with { replace: true }
  - same-URL click scrolls without pushing a new history entry
  - onBeforePageLoad / onAfterPageLoad fire
  - onBeforePageLoad returning false cancels page load
  - links inside .vp-raw trigger a full reload
  - external / download / target / non-HTML links are not intercepted
  - clicks on buttons inside links are not intercepted
  - route.path / route.hash / route.query stay in sync
  - unknown paths fall back to the 404 page
  - route.data.relativePath is populated from the loaded page

A small init-script-installed `resolveRouterOrThrow()` finds the router
via the Vue app's provides, so `page.evaluate` call sites stay terse and
typed against the public `Router` interface — no `any` in test code.

To support the flag, `hasNavigationApi()` now honours
`window.__VP_USE_LEGACY_ROUTER__` as a forced opt-out; this is documented
in-source as a test-only hook.

Author: brc-dd