Skip placeholder-coordinate postcodes (lat >= 90) in ONSPD import

ONS marks non-geographic postcodes (no grid reference) with a placeholder
latitude of 99.999999. The "Latest Centroids" export shipped ~12,789 such
rows, which were imported as real postcodes pointing at lat 99.99 and would
poison nearest-station distance maths. Drop them at ingest, with a test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/Console/Commands/ImportPostcodes.php

@@ -109,6 +109,13 @@ final class ImportPostcodes extends Command
                     continue;
                 }
 
+                // ONS marks non-geographic postcodes (no grid reference) with a
+                // placeholder latitude of 99.999999 — drop them so they don't
+                // poison nearest-station distance maths with a bogus location.
+                if (abs((float) $lat) >= 90) {
+                    continue;
+                }
+
                 $pcd = strtoupper(preg_replace('/\s+/', '', (string) $row[$columns[$pcdColumn]]));
 
                 if ($pcd === '' || strlen($pcd) < 5) {