lib/encoding/utf8+test: Hare port (decoder / next / encoderune / runesz / utf8sz / validate)

Hoehrmann DFA from ref/hare/encoding/utf8/decodetable.ha flattened
to 1D [2048]i8 (task #20: 2D-array jagged cgen still pending);
encoderune takes a caller buffer matching lib/encoding/hex.encode;
done/more/invalid all spelled as plain void aliases per lib/io's
eof precedent. Surface ports decoder + decode + next + encoderune
+ runesz + utf8sz + validate from ref/hare/encoding/utf8/{types,
decode,encode,rune}.ha. next() polarity rewritten from Hare's
`(state-1):uint >> 31` to an explicit `if state == 0` branch
because ww's uint is 64-bit (cmd/wcc/type.c:58); same effect, no
hidden 32-bit assumption.

Deferred (no in-tree callers): prev, slice, position, remaining,
appendrune, strencode, strdecode. String iteration (chars/
newchars/nextchar in the session-4 draft) dropped per Hare
discipline — belongs in lib/strings::iterator, not encoding/utf8.

Tests:
  - 968_utf8_run drives lib/encoding/utf8/utf8test.ww via ww run.
    21 @test fns: boundaries (ASCII, 2-byte, 3-byte, 4-byte
    encode/decode), surrogate/overlong/out-of-range/bad-continuation
    reject, max-in-range (U+10FFFF) accept, truncated→more, done@EOI,
    validate empty/mixed/malformed, encode/decode roundtrip. Two
    rows ported from ref/hare/encoding/utf8/decode.ha @test that
    were missing in the session-4 draft: bad-continuation
    [0xC2,0xFF]→invalid and max-in-range [0xF4,0x8F,0xBF,0xBF]→
    U+10FFFF.
  - 9xx stdlib runtime slot range extended from 970-989 to 960-989
    to accommodate utf8 at 968 (970-989 block was full).

90/90 ok. 995_self_rebuild stays green (ww2==ww3==ww4 byte-id).

test/wcc/968_utf8_run.c

@@ -0,0 +1,51 @@
+/*
+ * 968_utf8_run — execute the lib/encoding/utf8 @test fixture under the
+ * C-side `ww run` driver and assert exit 0.
+ *
+ * Thin wrapper mirroring 979_hex_run / 977_time_run: utf8test.ww carries
+ * its own `export fn main()` that drives the @test fns and signals which
+ * case failed via the exit code (signalled + 10).
+ *
+ * Slot 968: stdlib block 970–989 was full at landing time.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/wait.h>
+
+static int
+runwait(const char *cmd)
+{
+	int rc = system(cmd);
+	if (rc == -1) return -1;
+	if (WIFEXITED(rc)) return WEXITSTATUS(rc);
+	return 1;
+}
+
+int
+main(void)
+{
+	const char *bin = getenv("BIN");
+	if (!bin) bin = "out/bin";
+	char absbin[1024];
+	if (bin[0] != '/') {
+		char cwd[1024];
+		if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+		snprintf(absbin, sizeof absbin, "%s/%s", cwd, bin);
+		bin = absbin;
+	}
+	char cwd[1024];
+	if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+
+	const char *src = "lib/encoding/utf8/utf8test.ww";
+	char path[1024], cmd[2048];
+	snprintf(path, sizeof path, "%s/%s", cwd, src);
+	snprintf(cmd, sizeof cmd, "%s/ww run %s", bin, path);
+	int rc = runwait(cmd);
+	if (rc != 0) {
+		fprintf(stderr, "utf8_run FAIL: %s exited %d\n", src, rc);
+		return 1;
+	}
+	printf("utf8_run: %s ok\n", src);
+	return 0;
+}