lib/encoding/utf8+test: add strerror per Hare

lib/encoding/utf8/utf8.ww

@@ -29,6 +29,12 @@ export type more = void;
 // ref/hare/encoding/utf8/types.ha:9 — invalid UTF-8 sequence.
 export type invalid = !void;
 
+// ref/hare/encoding/utf8/types.ha:12 — fixed message; `invalid` carries
+// no payload, so the rendering is constant.
+export fn strerror(err: invalid) str = {
+	return "Invalid UTF-8";
+};
+
 // `done` is not a built-in singleton in ww (Hare ships it as part of
 // the type system). Plain `void` (not `!void`): end-of-input is a
 // continuation signal, not an error. lib/io spells its EOF the same

lib/encoding/utf8/utf8test.ww

@@ -12,6 +12,16 @@ import os;
 let signalled: i32 = 0;
 fn fail() void = { os.exit(signalled + 10); };
 
+fn streq(a: str, b: str) bool = {
+	if (a.len != b.len) { return false; };
+	let i: i32 = 0;
+	for (i < a.len) {
+		if (a[i] != b[i]) { return false; };
+		i += 1;
+	};
+	return true;
+};
+
 // ---- runesz: byte length per range ------------------------------------
 // ref/hare/encoding/utf8/rune.ha:5. Boundaries: 0x7F → 1, 0x80 → 2,
 // 0x7FF → 2, 0x800 → 3, 0xFFFF → 3, 0x10000 → 4, 0x10FFFF → 4.
@@ -575,6 +585,15 @@ fn fail() void = { os.exit(signalled + 10); };
 	if (utf8.remaining(&d1).len != 0) { fail(); };
 };
 
+// ---- strerror: constant rendering -------------------------------------
+// ref/hare/encoding/utf8/types.ha:12. `invalid` is payload-free; the
+// renderer collapses to a single fixture row.
+
+@test fn strerror_cases() void = {
+	let e: utf8.invalid;
+	if (!streq(utf8.strerror(e), "Invalid UTF-8")) { fail(); };
+};
+
 // ---- round-trip: encode → decode → equal rune --------------------------
 
 @test fn roundtrip() void = {
@@ -636,5 +655,6 @@ export fn main() i32 = {
 	signalled = 33; prev_max_in_range();
 	signalled = 34; prev_min_out_of_range();
 	signalled = 35; remaining_slice_position();
+	signalled = 36; strerror_cases();
 	return 0;
 };