Scope demo route trees with createRootAtom instead of a rootAtom prop

[randomdevpete]

Finding

Investigated scoping a demo's locationAtom to a path prefix via a jotai store context, so demos could declare route atoms as plain static values instead of a rootAtom prop + createXRoutes(root) factory. Not feasible with a store context, for two independent reasons (written up in packages/jarl-atoms/DESIGN-NOTES.md):

• A nested <Provider store={createStore()}> doesn't inherit from its parent — the subtree gets its own atomWithLocation, which only refreshes on popstate. In-subtree navigation calls history.pushState, which fires no popstate, so the outer store goes stale: the URL changes but every route atom outside the subtree still matches the old location.
• A store that shares state with its parent but overrides just locationAtom requires jotai 2.20's private, revision-numbered INTERNAL_buildStoreRev3 API (what jotai-scope is built on) — the public unstable_derive route is gone in this version. Not something a router should depend on.

createRootAtom({ basePath }) already gets the useful half of the idea in one store: route atoms below it are static module-level values, and the prefix is named once, on the root, where reverse() can prepend it again.

Change

• DataGridApp and BlogRoutingApp now call createRootAtom({ basePath: "/demos/..." }) at module scope and declare their route atoms as plain static values — no more rootAtom prop, no createXRoutes factory, no per-instance useMemo.
• Changelog was the one other place still on the old prop-passing pattern (same shape, one param route under a static mount). Migrated it too, since it's the same small change and leaves no straggler.
• Two stale comments in routes.ts corrected to describe the new mounting scheme.
• packages/jarl-atoms/DESIGN-NOTES.md — the project's existing home for recording rejected designs (predates this diff, already held similar entries from the v2 atoms core work) — gets a new section for this investigation.

Style-guide exceptions

None.