Link POI popups to offline Wikipedia via kiwix-serve

lib/maptool/kiwix.rb: a small reachability check for an external
kiwix-serve instance. GET /api/kiwix reports it (same optional-
integration pattern as offline-tiles/terrain-tiles); the frontend
adds a "Nearby on Wikipedia" link to a point's popup pointing straight
at kiwix-serve's own geo search (GET /search?latitude=&longitude=
&distance=, confirmed from libkiwix's actual source rather than
guessed) — linked to directly, not proxied/embedded, since it already
has a full search UI of its own.

docker-compose.yaml: new `kiwix` service (ghcr.io/kiwix/kiwix-serve)
on the default network, KIWIX_URL wired to reach it by service name.
The ZIM itself isn't part of the repo (large, self-contained archive)
— data/kiwix/ gitignored same as the *.pmtiles files, setup steps in
docs/development.adoc.

No kiwix-serve/ZIM available here to test end-to-end — verified the
app side against a stand-in local HTTP server (GET /api/kiwix flips
available correctly) and confirmed the generated popup link matches
the real geo-search query shape. docker-compose.yaml validated with
`docker-compose config`. Full test suite: 118 runs, 0 failures.

docs/deployment.adoc

@@ -35,9 +35,11 @@ expect this to flip the other way, `/srv/docker` symlinked to
   below) — `Dockerfile`, `docker-compose.yaml`, application code.
   `podman compose` reads the same `docker-compose.yaml` — no
   Podman-specific compose file needed.
-* `data/`, *not* part of that sync — uploaded GPX files and
-  `offline.pmtiles`. Persisted via a bind mount (`./data:/app/data` in
-  `docker-compose.yaml`), so a redeploy never touches it.
+* `data/`, *not* part of that sync — uploaded GPX files,
+  `offline.pmtiles`, `terrain.pmtiles`, and `kiwix/wikipedia.zim` (see
+  `docs/development.adoc` for each). Persisted via a bind mount
+  (`./data:/app/data` in `docker-compose.yaml`), so a redeploy never
+  touches it.
 
 == The native deploy runner