Enforce exhaustive matches for AST nodes

This commit addresses the critical rule regarding exhaustive `match`
expressions on AST node kinds. By explicitly listing all variants
instead of using wildcards (`_ =>`), we ensure that new AST node types
are consciously handled throughout the compilation pipeline. This
practice prevents subtle bugs and makes the compiler more robust against
future changes.

The `CLAUDE.md` file has been updated to reflect this critical rule.

The deleted `BUG_program_node_typeinference.md` file indicates that a
previously identified bug related to type inference with `Program` nodes
has been resolved, likely as a consequence of enforcing these exhaustive
matches.

CLAUDE.md

@@ -32,6 +32,7 @@
 ## Rust
 
 * Performance: Bevorzuge `Rc<dyn Trait>` + lokales `downcast_ref` gegenüber Deep Copies via `Rc::new(obj.clone())`.
+* CRITICAL: Keine Wildcards (`_ =>`, `other => other`) in `match`-Ausdrücken auf `NodeKind` oder `SyntaxKind`. Alle Varianten müssen explizit aufgeführt werden. Exhaustive Matches erzwingen, dass neue Varianten an jeder Stelle bewusst behandelt werden.
 
 ## Interaktion