Initial commit: ws_vatcheck at v1.0.3

Includes the fix for the GR/EL VAT prefix bug that broke exemption for
every Greek VAT number (in_array-on-values vs array_key_exists in
CacheTaxManager/VatNumber, plus the missing use import in
EuValidatorAdapter). Adds IMPROVEMENTS.txt with a checklist of features
adopted from comparing against the vatchecker module (offline-fallback
policy, failure logging, company-name check, configurable EU country
list, per-product exclusion, GB/XI mapping, AJAX live validation,
address-priority strategy).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

config.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<module>
+    <name>ws_vatcheck</name>
+    <displayName><![CDATA[Vat Check]]></displayName>
+    <version><![CDATA[1.0.3]]></version>
+    <description><![CDATA[Gecachte vat check]]></description>
+    <author><![CDATA[Johan van der Klis | De Websmid BV]]></author>
+    <tab><![CDATA[]]></tab>
+    <is_configurable>0</is_configurable>
+    <need_instance>0</need_instance>
+</module>