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>

tests/Feature/Console/ImportPostcodesTest.php

@@ -63,6 +63,21 @@ CSV;
         ->and(Postcode::find('BT11AA'))->toBeNull();
 });
 
+it('skips postcodes with placeholder coordinates (no grid reference)', function (): void {
+    $csv = <<<'CSV'
+pcd,pcds,doterm,lat,long
+"SW1A1AA","SW1A 1AA","",51.501009,-0.141588
+"GIR0AA","GIR 0AA","",99.999999,0.000000
+CSV;
+
+    $path = writeOnspdFixture($csv);
+
+    $this->artisan('postcodes:import', ['--file' => $path])->assertSuccessful();
+
+    expect(Postcode::count())->toBe(1)
+        ->and(Postcode::find('GIR0AA'))->toBeNull();
+});
+
 it('accepts ArcGIS ONSPD exports that use PCD7 instead of PCD', function (): void {
     $csv = <<<'CSV'
 OBJECTID,PCD7,PCD8,PCDS,DOTERM,LAT,LONG,x,y