wcc/check: reject self-import, both stages (#16 ENFORCE-checker)

check-(c): a package importing itself (any spelling) is a hard error,
mirroring Go. Predicate is leaf==owner at the N_USE/installdecl seam —
sound only after the PREP commits (dotted-test renames, package-less
boundary directive). Identical wording both stages; diagnostics-only,
byte-id-neutral. 948 pins the reject in both compilers; 708's
pos_selfimp (which pinned the abolished self-import skip) converts to
neg_selfimp + new pos_crossmod preserving the param-shadow tolerance
the case existed for. Checks (a) unused and (b)/(d) name-membership
stay deferred to the multi-package arc: imports are filename-keyed
pulls, so those need import->file provenance this compiler lacks.

test/wcc/948_selfimport.c

@@ -0,0 +1,89 @@
+/*
+ * 948_selfimport — #16 check-(c): a package may not import itself.
+ *
+ * Both compiler stages (w6c, w6c_ww) must loud-reject `package foo;
+ * import foo;` — nonzero exit AND a "self-import" diagnostic on stderr.
+ * Compile-only (`-o /dev/null`): w6c/w6c_ww do not expand imports (the
+ * driver does), so a lone fixture triggers check-(c) directly. This is
+ * NOT a ww_ww DRIVER test — it never invokes the driver — so it is
+ * Phase-1-safe and does not race the 990-997 byte-id gates (rule 14).
+ *
+ * check-(a) unused + (b)/(d) membership are DEFERRED to task #8 (ww's
+ * filename-keyed file-inclusion imports lack import->file->symbol
+ * provenance); only self-import is package-model-independent.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+static const char *
+absbin(void)
+{
+	const char *b = getenv("BIN");
+	if (!b) b = "out/bin";
+	if (b[0] == '/') return b;
+	static char buf[2048];
+	char cwd[1024];
+	if (getcwd(cwd, sizeof cwd) == NULL) return NULL;
+	snprintf(buf, sizeof buf, "%s/%s", cwd, b);
+	return buf;
+}
+
+/* reject — `<comp> <fixture> -o /dev/null` must exit nonzero and print
+ * <substr> on stderr. */
+static int
+reject(const char *bin, const char *comp, const char *fixture,
+    const char *substr)
+{
+	int pid = getpid();
+	char errf[256], cmd[4096], line[4096];
+	snprintf(errf, sizeof errf, "/tmp/si948_%s_%d.err", comp, pid);
+	snprintf(cmd, sizeof cmd, "%s/%s %s -o /dev/null 2>%s",
+	    bin, comp, fixture, errf);
+
+	int rc = system(cmd);
+	if (rc == 0) {
+		fprintf(stderr, "948 FAIL: %s accepted %s (expected reject)\n",
+		    comp, fixture);
+		return 1;
+	}
+
+	int found = 0;
+	FILE *f = fopen(errf, "r");
+	if (f) {
+		while (fgets(line, sizeof line, f))
+			if (strstr(line, substr)) { found = 1; break; }
+		fclose(f);
+	}
+	unlink(errf);
+	if (!found) {
+		fprintf(stderr, "948 FAIL: %s rejected %s but no '%s' on stderr\n",
+		    comp, fixture, substr);
+		return 1;
+	}
+	return 0;
+}
+
+static const struct {
+	const char *comp;
+	const char *fixture;
+	const char *substr;
+} rows[] = {
+	{ "w6c",    "test/wcc/data/selfimport.ww", "self-import" },
+	{ "w6c_ww", "test/wcc/data/selfimport.ww", "self-import" },
+};
+
+int
+main(void)
+{
+	const char *bin = absbin();
+	if (!bin) { fprintf(stderr, "948 FAIL: getcwd\n"); return 1; }
+
+	for (size_t i = 0; i < sizeof rows / sizeof rows[0]; i++)
+		if (reject(bin, rows[i].comp, rows[i].fixture, rows[i].substr) != 0)
+			return 1;
+
+	printf("self-import rejected by w6c and w6c_ww\n");
+	return 0;
+}