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/parser.bux

@@ -704,6 +704,19 @@ func parserParseStmt(p: *Parser) -> *Stmt {
         return s;
     }
 
+    // defer expr
+    if kind == tkDefer {
+        discard parserAdvance(p);
+        let val: *Expr = parserParseExpr(p);
+        parserMatch(p, tkSemicolon);
+        let s: *Stmt = bux_alloc(sizeof(Stmt)) as *Stmt;
+        s.kind = skDefer;
+        s.line = line;
+        s.column = col;
+        s.child1 = val;
+        return s;
+    }
+
     // if
     if kind == tkIf {
         discard parserAdvance(p);