feat(docs): 678 — routing demo that switches on a page route atom

[randomdevpete]

Summary

A deliberately minimal routing demo: one route atom holds which of a fixed set of pages the URL
names, a plain switch on its value picks the component to render, and the nav buttons navigate by
setting that atom instead of rendering <Link>s. It imports nothing from jarl-react — no Link,
Route or Switch component appears anywhere in SwitchRoutingApp.tsx.

The setRouteAtom question

The ticket left open whether to add a setRouteAtom primitive to jarl-atoms or approximate the
same shape with existing pieces. This PR approximates it: the fixed-set page segment is a
paramRouteAtom narrowed by a transformRouteAtom, the same composition numericRouteAtom's own
docstring calls "the usual way to build a constrained segment from the existing primitives". The
explicit generics on that transformRouteAtom call (<{ page: string }, { page: SwitchRoutingPage }>) are load-bearing: they pin the atom's value to the SwitchRoutingPage union rather than letting
it widen to string, which is what lets the switch in pageView be exhaustive with no default
case. Confirmed by typechecking the branch directly (tsc reports no errors on the new files, and the
same pre-existing, unrelated errors — missing @emotion/styled/highlight.js types, a generated
source-files.json — appear identically on the base branch).

No new export was added to jarl-atoms. setRouteAtom collides with jotai's own write vocabulary —
this demo already calls useSetAtom(pageRoute), and useSetAtom(setRouteAtom(...)) would fight
itself — and the base branch (#97) already composes constrained segments the same way, locally, for
its own demo (dateSegmentRouteAtom/filenameRouteAtom in ComplexRoutingApp.tsx, neither
exported).

Ticket 927 tracks a properly-named oneOfRouteAtom primitive for jarl-atoms, filed to land once
789 does.

Stack

Stacked on #97 (ticket 677), which this ticket depends on. Targets
task-677-complex-routing-demo-custom-path-atoms, not master.