Translate remaining German content to English

Finishes the project-wide English convention:
- CLAUDE.md (the user opted in to translate it too).
- examples/hello.ail.json: "Hallo, AILang." -> "Hello, AILang.".
- E2E test assertion and IR snapshot for hello regenerated to match.
- DESIGN.md "Project language: English" section: the previous
  CLAUDE.md exception is dropped.

After this commit, no German remains in any committed file
(grep -P '[äöüÄÖÜß]' is empty across .rs / .md / .json / .ll).

Verified: cargo test --workspace passes (44/44).

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

examples/hello.ail.json

@@ -17,7 +17,7 @@
         "t": "do",
         "op": "io/print_str",
         "args": [
-          { "t": "lit", "lit": { "kind": "str", "value": "Hallo, AILang." } }
+          { "t": "lit", "lit": { "kind": "str", "value": "Hello, AILang." } }
         ]
       }
     }