lib+test: fmt rename fprint→fdprint; add io.stream fprint sink

lib/CLAUDE.md

@@ -29,7 +29,7 @@ Signatures mirror Hare too, modulo:
 - Call-site variadic sugar matches Hare. `fn f(args: T...)` declares
   a Hare-style variadic; call sites either gather N args into a
   fresh `[]T` (`fmt.println(42, "hi", true)`) or forward an existing
-  slice with `xs...` (`fprintln(fd, args...)`). The bare `T...` form
+  slice with `xs...` (`fdprintln(fd, args...)`). The bare `T...` form
   in tagged unions still means spread-flatten (`(...inner | E)`);
   the two uses don't overlap because `T...` only attaches to a
   *param* decl. `lib/fmt` is intentionally print-string-only — no