feat(jarl-atoms): 927 — enumRouteAtom binds a path segment from a fixed set of values

[randomdevpete]

Adds enumRouteAtom, a route primitive that binds one path segment to a value drawn from a fixed set, typed as the union of that set rather than as string. Adopts it on the docs site's /docs and /api routes.

• Decide the name. It landed as enumRouteAtom, not the ticket's working oneOfRouteAtom. enum is the ecosystem's word for a value drawn from a fixed set of strings (JSON Schema/OpenAPI enum, z.enum), and it keeps the segment constructors named for the segment kind: static, param, numeric, enum. "one of" names a choice between whole alternatives, which is exactly what PR feat(jarl-atoms)!: 420 — unionRouteAtom, and notAtom taking one route plus options #110's unionRouteAtom does — two near-synonyms in one export list invite reaching for the wrong one. One commit to rename if OWNER prefers the original.
• It is complementary to feat(jarl-atoms)!: 420 — unionRouteAtom, and notAtom taking one route plus options #110's unionRouteAtom, not overlapping. A union of static routes binds no values (RouteValues of a static route is {}), so it can be neither switched on nor navigated by value. Neither subsumes the other.
• No precedence rule to get wrong, unlike feat(jarl-atoms)!: 420 — unionRouteAtom, and notAtom taking one route plus options #110: a path segment is one string, so at most one member matches and list order decides nothing.
• The /docs and /api adoption drops two unchecked as casts and turns an unknown guide name from a 200-with-a-not-found-body into a real 404.

The ticket's second "Done when" — simplifying ticket 678's demo — is not doable on this base, because SwitchRoutingApp.tsx lives only on origin/task-678-very-simple-logic-switch-routing-demo, a separate stack off master. Whichever lands second collapses that demo's paramRouteAtom + transformRouteAtom + isSwitchRoutingPage into one enumRouteAtom call — a five-line deletion.