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/token.bux

@@ -139,6 +139,9 @@ const tkAwait: int = 103;
 const tkSpawn: int = 104;
 const tkDiscard: int = 105;
 const tkDefer: int = 106;
+const tkSwitch: int = 107;
+const tkCase: int = 108;
+const tkDefault: int = 109;
 
 // ---------------------------------------------------------------------------
 // Token struct