Client-side navigation between hydrated pages

Purpose

Move between hydrated pages without a full document load (GitHub issue #37: console screens realm → clients → client). Builds on the page loader() and bootstrap payload — no second data API.

Public surface

import { Link, navigate, prefetch } from "patties/client";

<Link href="/realms/acme/clients">Clients</Link>   // prefetch="intent" by default
await navigate("/realms/acme", { replace: true });

Link renders a plain <a>, so it SSRs and degrades to normal navigation on zero-JS and island pages.

Behavior

1. A request carrying x-patties-nav: 1 to a page route runs middleware and loader() as usual but answers JSON instead of HTML: { hydrated: true, route, script, bootstrap: { params, data? }, title? }, or { hydrated: false } for a route without a page bundle.
2. Link intercepts unmodified primary clicks (no modifier keys, no target/download) when the document booted a hydrated page and the target is same-origin. Hover/focus warms the payload and chunk (prefetch).
3. navigate fetches the payload (following redirects — the final URL goes in the address bar), import()s the page chunk unless already registered, pushes history, sets document.title from meta.title, and calls root.render(<Page key={route} {...props} />) on the existing root. Moving between routes remounts; a param change within one route keeps state.
4. popstate re-runs navigate without touching history; scroll goes to the hash target or the top on push.
5. Anything unhandled — hydrated: false, a non-JSON or non-OK response, a chunk load failure — falls back to location.assign, so navigation always succeeds.

Non-goals

head() nodes other than the title are not swapped on soft navigation; nested layouts and parallel data loading are future work.

Tests

tests/client/page-runtime.test.tsx, tests/integration/hydrate.test.ts (nav payloads on bun + edge).