docs: translate all code comments to English (language policy)

Translate every Italian /// XML doc comment, <!-- --> XAML comment, and
// inline comment to English across all source files (Core, App, tests).
Add the language policy to CLAUDE.md (conversation Italian; all code
and docs English). Update one test assertion that checked an Italian
exception message that was also translated.

src/Core/Crypto/Slip132.cs

@@ -20,8 +20,8 @@ public static class Slip132
         Encoders.Base58Check.EncodeData([.. profile.ExtKeyHeaders[kind].PrivateBytes(), .. key.ToBytes()]);
 
     /// <summary>
-    /// Riconosce l'header (→ ScriptKind) e decodifica una chiave pubblica estesa:
-    /// è la via dell'import watch-only (§4.4).
+    /// Recognises the header (→ ScriptKind) and decodes an extended public key:
+    /// this is the watch-only import path (§4.4).
     /// </summary>
     public static bool TryDecodePublic(string encoded, ChainProfile profile,
         out ExtPubKey? key, out ScriptKind kind)