feat: full match expressions in bootstrap and selfhost

Lower match to if-else for literals, ranges, enum tags, and wildcards.
Fix bootstrap literal arms that always matched; port real arm AST/parser
and Lcx_LowerMatch to selfhost with last-expression return. Expand
pattern_matching example and add parse/use-after-move/double-mut golden
diagnostics. Selfhost-loop remains binary-identical.

tests/error_golden/use_after_move/src/Main.bux

@@ -0,0 +1,12 @@
+@[Checked]
+func Take(s: own String) -> own String {
+    return s;
+}
+
+@[Checked]
+func Main() -> int {
+    let a: own String = "hello";
+    let b: own String = Take(a);
+    let c: own String = a;
+    return 0;
+}