lib/encoding/hex+test: Hare port (encode / decode / sizes)

Replaces the 19-line placeholder. Five entrypoints per
ref/hare/encoding/hex/hex.ha + README:13:

- invalid (!void) — mirrors errors::invalid (hex.ha:175 decodestr).
  base32's !i32 is a pre-existing in-tree divergence; hex doesn't
  carry it forward.
- encodedsize(n) = n*2 — derived from hex.ha:46-55 encode_writer
  lowercase 2-chars-per-byte.
- decodedsize(n) = n/2 — inverse; hex.ha:158.
- encode(dst, src) i32 — lowercase output per README:13 + hex.ha:91.
- decode(dst, src) (i32 | invalid) — accepts lower / upper / mixed;
  returns invalid on odd length (hex.ha:154) or non-hex char
  (hex.ha:161-163).

Deferred (cite-and-defer, same pattern as base32/base64):
- newencoder / newdecoder — Hare's io::handle stream API; ww has no
  io::handle integration yet.
- encodestr / decodestr — allocator-returning sum-result; needs
  os.alloc-backed memio dynamic, not wired.
- dump — hexdump-with-ASCII view; needs io::handle + fmt::fprintf
  into a write sink.

Test: lib/encoding/hex/hextest.ww 13 rows — sizes, encode_basic
(Hare's CAFEBABEDEADF00D verbatim), encode_zero / encode_ff /
encode_empty (nibble corners + sign-extend + table off-by-one),
decode_{lower,upper,mixed} (case acceptance), decode_{empty,
odd_length,bad_char,bad_char_mid} (error paths), roundtrip_all_bytes
(0..255 full nibble+shift family — Class B exerciser).

Driver test/wcc/979_hex_run.c slotted between 978_intdiv_signed and
980_memio_run.

test/wcc/979_hex_run.c

@@ -0,0 +1,49 @@
+/*
+ * 979_hex_run — execute the lib/encoding/hex @test fixture under the
+ * C-side `ww run` driver and assert exit 0.
+ *
+ * Same thin-wrapper shape as 983_base32_run / 977_time_run:
+ * hextest.ww carries its own `export fn main()` that drives the
+ * @test fns and signals which case failed via the exit code.
+ */
+#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/hex/hextest.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, "hex_run FAIL: %s exited %d\n", src, rc);
+		return 1;
+	}
+	printf("hex_run: %s ok\n", src);
+	return 0;
+}