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/hir.bux

@@ -33,6 +33,7 @@ const hTupleInit: int = 27;
 const hMatch: int = 28;
 const hSpawn: int = 29;
 const hAwait: int = 30;
+const hDefer: int = 31;
 
 // ---------------------------------------------------------------------------
 // HirArgList — linked list for call arguments beyond 2