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/bux.toml

@@ -0,0 +1,7 @@
+[Package]
+Name    = "double_mut_borrow"
+Version = "0.1.0"
+Type    = "bin"
+
+[Build]
+Output = "Bin"