Add scroll restoration to the preact-router Navigation

[lemonmade]

Summary

Navigation now manages scroll position across navigations, fixing the
common SPA bug where client-side navigation lands at the wrong scroll offset.

The browser's native scroll restoration runs against the document as it exists
at popstate time, but an async route hasn't rendered its content yet — so a
back/forward restore lands against the wrong (usually shorter) document. Forward
navigations are also frequently left at the previous page's offset rather than
the top.

In the browser, Navigation now:

• sets history.scrollRestoration = 'manual' and keeps its own per-entry scroll
  offsets, keyed by navigation id and persisted to sessionStorage (so they
  survive a reload within the tab session, capped at 50 entries);
• forward navigation → resets to the top, or scrolls to the URL hash target
  when present;
• back/forward navigation → restores the offset the entry was last left at;
• reload → restores the offset of the entry it lands on.

Restores that need the destination route's content committed (a saved offset or
a hash target) are applied on the next animation frame; a plain reset to the top
is applied synchronously to avoid a flash of the new route at the previous
offset. All of this is window/document-scoped and fully guarded for non-browser
(SSR) construction.

Enabled by default. Pass scrollRestoration: false to leave scrolling to the
browser/your app:

const navigation = new Navigation(initialURL, {scrollRestoration: false});

A changeset (minor bump for @quilted/preact-router) is included.

Test plan

• Added Navigation scroll-restoration unit tests (jsdom): manual mode by
  default, opt-out, reset-to-top on forward nav, restore-on-back, and
  sessionStorage persistence keyed by navigation id.
• tsc --build packages/preact-router/tsconfig.json passes.
• prettier --check clean.
• pnpm test in CI (the equivalent suite was validated against the built
  output in a downstream consumer; CI builds the workspace tooling first).