fix(datacenters): report newly tagged features osmium cannot export

A still-tagged feature without exportable geometry already keeps its old
line. A newly tagged one had nowhere to go and was dropped silently; it is
now listed as skipped with a warning, so a refresh never loses a site or
hides a broken one.

src/data/datacentersNorwayRefresh.test.mjs

@@ -52,9 +52,10 @@ def line(osm_id, sequence):
                                       'type': 'data_center', 'osm_id': osm_id}})
 norway = [([(0, 0), (20, 0), (20, 80), (0, 80)], False)]
 lines = [line(7, 1), line(8, 2)]
-out, report = merge(lines, {}, norway, {'w7'})
+out, report = merge(lines, {}, norway, {'w7', 'w12'})
 assert out == [lines[0]], out
 assert report['unexported'] == ['w7'] and report['removed'] == ['w8'], report
+assert report['skipped'] == ['w12'] and report['added'] == [], report
 `,
   );
 });

src/data/local_data/datacenters/README.md

@@ -29,7 +29,8 @@ reflects OSM as of the extract below, everything else the original snapshot.
 - Contact tags are dropped as above; unchanged features keep their original
   line, so geometry is only rewritten when tags or rings actually changed.
   A feature is removed only once it is no longer tagged; one still tagged but
-  without an exportable geometry keeps its line and is reported as unexported.
+  without an exportable geometry keeps its line and is reported as unexported,
+  and a newly tagged one osmium cannot export is reported as skipped.
 - Refresh: `python3 scripts/build-datacenters-norway.py --cache <dir>` uses
   Geofabrik's current Norway extract and prints the dated name it used; record
   that name, its OSM timestamp and MD5 here. Requires `osmium-tool`.