Add a persisted test suite

`bundle exec rake test` (minitest, a default gem but pinned in
Gemfile for reproducibility) now covers what ad-hoc scripts kept
re-verifying by hand during development: Gpx/Fit/TrackStats parsing,
GpxStore/LiveTrackStore file/TTL logic, LiveTrackClient's payload
parsing, and app.rb's routes end to end via Rack::MockRequest.

MAPTOOL_DATA_DIR (new, app.rb) lets the suite point GPX_STORE/
LIVETRACK_STORE at a throwaway directory instead of real data/ —
without it every run would leave uploaded test files behind. Each
lib file's old `if __FILE__ == $0` self-check is now redundant with
real tests covering the same ground, so removed rather than kept as
a second copy of the same assertions. Wired into ci.yml so it isn't
just a local convenience.

docs/development.adoc

@@ -91,13 +91,37 @@ gpsd -N -n tcp://127.0.0.1:5001
 `GET /api/gnss/position` should then report the fake fix (Bratislava
 Castle, in the sentences above) within a couple of seconds.
 
+== Tests
+
+`bundle exec rake test` runs the whole suite (`test/test_*.rb`,
+Minitest — a default gem, but pinned in the `Gemfile`'s `:test` group
+for reproducibility, same reasoning as `rake` itself). `test_app.rb`
+drives `app.rb`'s routes end to end through `Rack::MockRequest` rather
+than a real server; everything else tests one `lib/maptool/` module in
+isolation. `MAPTOOL_DATA_DIR` (see `app.rb`) is pointed at a throwaway
+`Dir.mktmpdir` by `test/test_helper.rb` before `app.rb` loads, so
+running the suite never touches real `data/` — the app-level tests
+also wipe that directory between each test (see `AppTestHelper#teardown`),
+since `GPX_STORE`/`LIVETRACK_STORE` are process-wide constants shared
+by every test in the run.
+
+`LiveTrackClient`'s own tests use a synthetic fixture (no network) —
+its parsing was checked against real Garmin sessions by hand during
+development, not by the suite, since that needs a live, unexpired
+session token to fetch. `Fit`'s tests likewise build a minimal FIT
+file by hand rather than shipping a real device fixture; correctness
+against real files (position/altitude fields, definition-message
+framing, compressed-timestamp headers) was checked by hand against
+`python-fitparse`'s test fixtures during development — see
+`docs/architecture.adoc`.
+
 == Continuous integration
 
 `.gitea/workflows/ci.yml` runs on every push/PR against a self-hosted
 Gitea Actions runner: `ruby -c` on every `.rb` file, a `require_relative
-"app"` smoke check that the app loads without error, and `node --check`
-on every file under `public/js/`. There is no test suite yet — these
-are cheap syntax/load checks, not behavioral tests.
+"app"` smoke check that the app loads without error, `bundle exec rake
+test` (see above), and `node --check` on every file under
+`public/js/` — no JS test suite, just a syntax check.
 
 `.gitea/workflows/renovate.yml` runs
 https://docs.renovatebot.com[Renovate] (the `renovate/renovate` image)