test/wcc: extend carrier ownership to the bootstrap and platform gates

Same contract as the 27 repaired byte/artifact survivors: checked
acquisition, one all-exit cleanup funnel per carrier, ENOENT-tolerant
checked unlinks, exact-path deletion, and cleanup failure fails a
passing carrier without overwriting its diagnostic. Also closes the
vacuous-green channels: empty-artifact pairs no longer byte-compare
equal, capture and staging failures fail their row loudly, 950 requires
a clean checker exit on no-error rows and hard-fails on a missing
wwdump_ww, 992 keys its scratch per invocation so a failed cleanup
cannot leak stale objects into the next tool's compare, and 995 no
longer pre-sweeps a workdir it never created. No assertion is weakened;
990_selfhost is retired in the next commit rather than repaired.

test/wcc/991_w6a_ww.c

@@ -14,6 +14,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <errno.h>
 #include <unistd.h>
 #include <glob.h>
 #include <sys/wait.h>
@@ -63,34 +64,47 @@ static int
 diff_one(const char *bin, const char *src)
 {
 	char co[64], wo[64], cmd[2048];
+	char *bc = NULL, *bw = NULL;
+	size_t nc = 0, nw = 0;
+	int rc = -1;
 	snprintf(co, sizeof co, "/tmp/wwa_%d_c.o", getpid());
 	snprintf(wo, sizeof wo, "/tmp/wwa_%d_w.o", getpid());
 
 	snprintf(cmd, sizeof cmd, "timeout 180 %s/w6a -o %s %s 2>/dev/null", bin, co, src);
 	if (runwait(cmd) != 0) {
 		fprintf(stderr, "w6a_ww FAIL: C w6a errored on %s\n", src);
-		unlink(co);
-		return -1;
+		goto cleanup;
 	}
 	snprintf(cmd, sizeof cmd, "timeout 180 %s/w6a_ww -o %s %s 2>/dev/null", bin, wo, src);
 	if (runwait(cmd) != 0) {
 		fprintf(stderr, "w6a_ww FAIL: ww w6a errored on %s\n", src);
-		unlink(co); unlink(wo);
-		return -1;
+		goto cleanup;
 	}
-	char *bc = NULL, *bw = NULL;
-	size_t nc = 0, nw = 0;
-	int rc = 0;
 	if (slurp(co, &bc, &nc) < 0 || slurp(wo, &bw, &nw) < 0) {
 		fprintf(stderr, "w6a_ww FAIL: cannot read .o for %s\n", src);
-		rc = -1;
+	} else if (nc == 0) {
+		fprintf(stderr, "w6a_ww FAIL: empty .o for %s\n", src);
 	} else if (nc != nw || memcmp(bc, bw, nc) != 0) {
 		fprintf(stderr, "w6a_ww FAIL: %s — C %zu vs ww %zu bytes\n",
 		    src, nc, nw);
-		rc = -1;
-	}
+	} else
+		rc = 0;
+
+cleanup:
 	free(bc); free(bw);
-	unlink(co); unlink(wo);
+	/* co/wo are reused across corpus files; a stale .o left behind
+	 * would be slurped as a later file's output. */
+	int cleanfail = 0;
+	if (unlink(co) != 0 && errno != ENOENT) {
+		perror(co);
+		cleanfail = 1;
+	}
+	if (unlink(wo) != 0 && errno != ENOENT) {
+		perror(wo);
+		cleanfail = 1;
+	}
+	if (cleanfail && rc == 0)
+		rc = -1;
 	return rc;
 }
 
@@ -112,6 +126,9 @@ main(void)
 	 * intentionally contains package main plus legacy file-import packages,
 	 * so it is not a directory-package root. */
 	char stem[256], cmd[4096];
+	int fail = 0;
+	int n = 0;
+	int gen = 0;
 	snprintf(stem, sizeof stem, "/tmp/wwa_%d_sh", getpid());
 	snprintf(cmd, sizeof cmd,
 	    "timeout 400 %s/ww build -o %s "
@@ -120,7 +137,9 @@ main(void)
 	if (runwait(cmd) != 0) {
 		fprintf(stderr,
 		    "w6a_ww FAIL: cannot sep-build selfhost/cmd/w6a/main.ww\n");
-		return 1;
+		/* a failed build can leave a partial stem/.sepwork tree. */
+		fail = 1;
+		goto cleanup;
 	}
 
 	/* hand-written runtime — small, covers MOVQ/LEAQ/CALL/SYSCALL/RET;
@@ -129,8 +148,6 @@ main(void)
 		"rt/start.s", "rt/syscall.s", "rt/abort.s",
 		"rt/alloc.s", "rt/streq.s", NULL,
 	};
-	int fail = 0;
-	int n = 0;
 	for (int i = 0; rtfiles[i]; i++) {
 		char p[2048];
 		snprintf(p, sizeof p, "%s/%s", cwd, rtfiles[i]);
@@ -152,17 +169,32 @@ main(void)
 			if (sz <= 0) continue;
 			if (diff_one(bin, g.gl_pathv[i]) != 0) fail++;
 			n++;
+			gen++;
 		}
 		globfree(&g);
 	}
+	/* the PASS banner claims generated-asm coverage; an empty glob
+	 * (or all-empty units) must not go green on the rt corpus alone. */
+	if (gen == 0) {
+		fprintf(stderr,
+		    "w6a_ww FAIL: no non-empty generated .s under %s.sepwork\n",
+		    stem);
+		fail++;
+	}
 
+cleanup:
 	snprintf(cmd, sizeof cmd, "rm -rf %s.sepwork %s", stem, stem);
-	if (system(cmd)) {}
+	int cleanfail = runwait(cmd) != 0;
+	if (cleanfail)
+		fprintf(stderr, "w6a_ww: cleanup of %s{,.sepwork} failed\n",
+		    stem);
 
 	if (fail) {
 		fprintf(stderr, "w6a_ww: %d/%d diff(s) failed\n", fail, n);
 		return 1;
 	}
+	if (cleanfail)
+		return 1;
 	printf("w6a_ww: byte-identical to C w6a on %d corpus files "
 	    "(rt/*.s + selfhost/cmd/w6a sep-built .s)\n", n);
 	return 0;