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>

vendor/.htaccess

@@ -0,0 +1,10 @@
+# Apache 2.2
+<IfModule !mod_authz_core.c>
+    Order deny,allow
+    Deny from all
+</IfModule>
+
+# Apache 2.4
+<IfModule mod_authz_core.c>
+    Require all denied
+</IfModule>