fix(postcodes/GB): null sentinel 99.999999 coords on Crown Dependencies (GY,IM,JE)

Guernsey, Isle of Man, and Jersey are absent from the dwyl/OS source
dataset, so the importer emitted lat=99.999999 as a sentinel. That's
outside the valid latitude range (-90..90) and trips the geo-bounds
validator. Null the coordinates so the codes stay queryable; the
country-level postal_code_format / regex still apply.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dr5hn

contributions/postcodes/GB.json

@@ -414,8 +414,8 @@
     "country_id": 232,
     "country_code": "GB",
     "locality_name": "Guernsey",
-    "latitude": "99.999999",
-    "longitude": "0.000000",
+    "latitude": null,
+    "longitude": null,
     "type": "area",
     "source": "ordnance-survey-via-dwyl"
   },
@@ -514,8 +514,8 @@
     "country_id": 232,
     "country_code": "GB",
     "locality_name": "Isle of Man",
-    "latitude": "99.999999",
-    "longitude": "0.000000",
+    "latitude": null,
+    "longitude": null,
     "type": "area",
     "source": "ordnance-survey-via-dwyl"
   },
@@ -544,8 +544,8 @@
     "country_id": 232,
     "country_code": "GB",
     "locality_name": "Jersey",
-    "latitude": "99.999999",
-    "longitude": "0.000000",
+    "latitude": null,
+    "longitude": null,
     "type": "area",
     "source": "ordnance-survey-via-dwyl"
   },