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/double_mut_borrow/src/Main.bux

@@ -0,0 +1,12 @@
+@[Checked]
+func TwoMut(a: &mut int, b: &mut int) {
+    *a = 1;
+    *b = 2;
+}
+
+@[Checked]
+func Main() -> int {
+    var x: int = 0;
+    TwoMut(&x, &x);
+    return 0;
+}