encoding/utf8: enforce scalar and buffer bounds

lib/encoding/utf8/encode_test.ww

@@ -4,6 +4,7 @@
 package utf8_test;
 
 import encoding.utf8;
+import test;
 
 // ref/hare/encoding/utf8/encode.ha:7. Byte vectors come from the
 // Unicode specification (UAX standard examples).
@@ -41,3 +42,11 @@ import encoding.utf8;
 	assert(!(out[2] != 0xA6u8));
 	assert(!(out[3] != 0x80u8));
 };
+
+// The ww API writes into caller storage, unlike Hare's static-buffer return.
+// A short slice must fail before the first out-of-bounds store.
+@test fn encode_short_buffer_aborts() void = {
+	test.expectabort();
+	let out: [1]u8;
+	utf8.encoderune(out[0:1], 0xE9u32: rune);
+};