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/token.bux

@@ -138,6 +138,7 @@ const tkAsync: int = 102;
 const tkAwait: int = 103;
 const tkSpawn: int = 104;
 const tkDiscard: int = 105;
+const tkDefer: int = 106;
 
 // ---------------------------------------------------------------------------
 // Token struct