feat: defer statement support in both compilers

- Add  statement for function-level deferred execution
- Deferred expressions run in LIFO order on function exit (return or implicit)
- Bootstrap: desugar defers in hir_lower.nim (inject before return + end of func)
- Selfhost: emit defers in c_backend.bux via CEmitter defer stack
- Both: add tkDefer token, skDefer AST node, hDefer HIR node
- Parser, lexer, sema, token files updated in both bootstrap/ and src/

src/ast.bux

@@ -157,6 +157,7 @@ const skReturn: int = 8;
 const skBreak: int = 9;
 const skContinue: int = 10;
 const skDecl: int = 11;
+const skDefer: int = 12;
 
 struct ElseIf {
     line: uint32;