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/expected.err

@@ -0,0 +1,6 @@
+error: type errors in project
+error: mutable borrow conflict: arguments 1 and 2 both borrow '&mut x'
+  --> FILE:10:11
+    |
+ 10 |     TwoMut(&x, &x);
+    |           ^