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/

bootstrap/ast.nim

@@ -238,6 +238,7 @@ type
     skStaticAssert
     skComptime
     skEmit
+    skDefer
     skDecl
 
   ElseIf* = object
@@ -298,6 +299,8 @@ type
     of skEmit:
       stmtEmitExpr*: Expr
       stmtEmitEvaluated*: string  ## filled by sema CTFE
+    of skDefer:
+      stmtDeferBody*: Expr
     of skDecl:
       stmtDecl*: Decl