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/lexer.bux

@@ -301,6 +301,7 @@ func lexKeywordKind(text: String) -> int {
     if String_Eq(text, "super") { return tkSuper; }
     if String_Eq(text, "sizeof") { return tkSizeOf; }
     if String_Eq(text, "discard") { return tkDiscard; }
+    if String_Eq(text, "defer") { return tkDefer; }
     if String_Eq(text, "async") { return tkAsync; }
     if String_Eq(text, "await") { return tkAwait; }
     if String_Eq(text, "spawn") { return tkSpawn; }