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/development.adoc

@@ -72,6 +72,32 @@ something the running app calls itself — drop the output into
 mapped as a way/relation, e.g. a supermarket traced as a building
 outline rather than a single point).
 
+== Offline Wikipedia (kiwix-serve)
+
+A point popup's "Nearby on Wikipedia" link needs
+https://github.com/kiwix/kiwix-tools[kiwix-serve] running with a
+Wikipedia https://library.kiwix.org[ZIM] loaded — set `KIWIX_URL` to
+wherever it's reachable (`docker-compose.yaml` already runs a `kiwix`
+service pointed at `data/kiwix/wikipedia.zim` and wires `KIWIX_URL` to
+it; blank/unset disables the link, same as every other optional
+integration here). Not part of the repo — a ZIM is a large,
+self-contained archive, obtain one locally:
+
+* https://library.kiwix.org/#lang=eng&category=wikipedia[Kiwix's own
+  library] has pre-built Wikipedia ZIMs — a `_maxi` variant per
+  language/region includes images, `_nopic` is much smaller if
+  browsing images offline doesn't matter here.
+* Download it to `data/kiwix/wikipedia.zim` (the exact filename
+  `docker-compose.yaml`'s `kiwix` service expects — rename after
+  downloading, Kiwix's own filenames are much more specific).
+
+Once it's there, `docker compose up -d kiwix` (or a full restart)
+picks it up — `GET /api/kiwix` reports `available: true` and the
+"Nearby on Wikipedia" link appears. kiwix-serve's own geo search
+(`GET /search?pattern=&latitude=&longitude=&distance=`,
+`lib/maptool/kiwix.rb`'s docstring) is linked to directly, not
+proxied/embedded — it already has a full search UI of its own.
+
 == GNSS position (gpsd)
 
 The "GNSS" control needs https://gpsd.io[gpsd] running and pointed at