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/parse_error/expected.err

@@ -0,0 +1,7 @@
+error: parse errors in FILE
+error: expected expression
+  --> FILE:2:13
+    |
+ 2 |     let x = ;
+    |             ^
+   = help: the previous statement may be incomplete (missing value or ';')