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

test/wcc/700_e2e.c

@@ -1639,7 +1639,7 @@ static const struct row rows[] = {
 	  "fn main() i32 = { return sumtag(1i64, \"hi\", true): i32; };", 42 },
 	/* Variadic forwarding: `wrap(args...)` passes the local slice
 	 * directly to `sum`, no re-gather. Mirrors Hare's wrapper shape
-	 * (`fn println(args: formattable...) = fprintln(os.stdout, args...)`). */
+	 * (`fn println(args: formattable...) = fdprintln(os.stdout, args...)`). */
 	{ "fn sum(args: i64...) i64 = {\n"
 	  "  let s: i64 = 0i64;\n"
 	  "  let i: i32 = 0;\n"
@@ -1654,8 +1654,8 @@ static const struct row rows[] = {
 	  "};", 42 },
 	/* lib/fmt user-side: `fmt.println(args: formattable...)` gathers
 	 * mixed-type args at the call site. End-to-end exercises the
-	 * lib/fmt graduation: the wrapper-chain `println → fprintln →
-	 * fprint` is itself variadic-forwarding, so this validates both
+	 * lib/fmt graduation: the wrapper-chain `println → fdprintln →
+	 * fdprint` is itself variadic-forwarding, so this validates both
 	 * gather (at main) and `args...` forward (inside lib/fmt). The
 	 * exit code is bytes printed (`hello 7\n` = 8). */
 	{ "use fmt;\n"