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>

crates/ail/tests/e2e.rs

@@ -54,7 +54,7 @@ fn max3_picks_largest() {
 #[test]
 fn hello_world_str_lit() {
     let stdout = build_and_run("hello.ail.json");
-    assert_eq!(stdout.trim(), "Hallo, AILang.");
+    assert_eq!(stdout.trim(), "Hello, AILang.");
 }
 
 /// Guards ADT codegen + match: recursive list, sum_list via match on Cons/Nil.

crates/ail/tests/snapshots/hello.ll

@@ -2,7 +2,7 @@
 source_filename = "hello.ail"
 target triple = "<NORMALIZED>"
 
-@.str_hello_str_0 = private unnamed_addr constant [15 x i8] c"Hallo, AILang.\00", align 1
+@.str_hello_str_0 = private unnamed_addr constant [15 x i8] c"Hello, AILang.\00", align 1
 
 declare i32 @printf(ptr, ...)
 declare i32 @puts(ptr)