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.

test/fixtures/sample.gpx

@@ -0,0 +1,26 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<gpx version="1.1" creator="test">
+  <wpt lat="48.1" lon="17.2">
+    <ele>200</ele>
+    <name>Start</name>
+    <desc>Trailhead, see https://example.com/trail for details</desc>
+    <time>2026-01-01T09:00:00Z</time>
+  </wpt>
+  <trk>
+    <name>Morning Loop</name>
+    <trkseg>
+      <trkpt lat="48.1" lon="17.2">
+        <ele>200</ele>
+        <time>2026-01-01T09:00:00Z</time>
+      </trkpt>
+      <trkpt lat="48.101" lon="17.201">
+        <ele>250</ele>
+        <time>2026-01-01T09:10:00Z</time>
+      </trkpt>
+      <trkpt lat="48.102" lon="17.202">
+        <ele>230</ele>
+        <time>2026-01-01T09:20:00Z</time>
+      </trkpt>
+    </trkseg>
+  </trk>
+</gpx>