feat: switch/case statement

- Add  syntax
- Desugars to if-else chain in HIR lowering (both compilers)
- No new HIR/C backend nodes needed — reuses hIf/hBinary/hBlock
- Supports integer, char, and enum tag dispatch
- Case body can be single statement (no braces required)

src/ast.bux

@@ -158,6 +158,7 @@ const skBreak: int = 9;
 const skContinue: int = 10;
 const skDecl: int = 11;
 const skDefer: int = 12;
+const skSwitch: int = 13;
 
 struct ElseIf {
     line: uint32;