refactor(legacy-ks2): use explicit year→URL mapping instead of base URL pattern

The file hosting uses non-deterministic URLs, so replace legacy_ks2_base_url
+ legacy_ks2_years with a single legacy_ks2_urls object mapping year codes
to download URLs. Configure the 4 pre-COVID years in meltano.yml.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pipeline/meltano.yml

@@ -23,6 +23,15 @@ plugins:
         - name: datasets
           kind: array
           description: List of EES dataset configs to extract
+        - name: legacy_ks2_urls
+          kind: object
+          description: "Year code → URL mapping for legacy KS2 CSVs"
+      config:
+        legacy_ks2_urls:
+          "201516": "http://10.0.1.224:8081/filebrowser/api/public/dl/R9jjXFWa?inline=true"
+          "201617": "http://10.0.1.224:8081/filebrowser/api/public/dl/tIwJPVQS?inline=true"
+          "201718": "http://10.0.1.224:8081/filebrowser/api/public/dl/GO7SKE0p?inline=true"
+          "201819": "http://10.0.1.224:8081/filebrowser/api/public/dl/jchDEHsv?inline=true"
 
     - name: tap-uk-ofsted
       namespace: uk_ofsted