test: contain sepwork scratch per-driver tmpdir, fix /tmp+in-repo leak (#8)

The wcc test drivers ran `ww build <bare-/tmp src>` with no -o, so the
compiler's <stem>.sepwork scratch landed beside the source and was never
cleaned: unbounded /tmp growth (2195 stale dirs observed) that fills tmpfs
and fabricates phantom test failures + silent harness aborts, and for
in-repo fixture builds leaked .sepwork into the tracked tree.

Each leaking build now writes its source + output inside a per-invocation
tmpdir, passes -o <tmpdir>/<stem> so the .sepwork lands inside it, and
rm -rf's the tmpdir on every exit path -- including fopen-fail and the
expected-fail reject builds (scratch is mkdir'd before the build can fail).
`ww run` and explicit-`-o`/byte-id helpers are left as-is; the 990/993
byte-id comparison logic is byte-for-byte unchanged.

Two items filed separately (this commit holds the no-Makefile / no-main.c
rail):
- #13: a stale <src>.s byte-id readback (749) silently no-ops since
  separate-compile emits .s to <ostem>.sepwork/__root.s; documented inline.
- #14: build-system Makefile recipes build selfhost/cmd/*/main.ww with no
  -o and leak main.sepwork in-tree (bounded, gitignored; own commit).

One concern -- sepwork leak hygiene -- across 228 drivers; uniform
transform applied per-file and two-round reviewed. make test: all 402
passed, zero net-new /tmp scratch, zero test-driven in-repo .sepwork.

test/wcc/975_dirs_toolong_run.c

@@ -15,7 +15,6 @@
  */
 #include <stdio.h>
 #include <stdlib.h>
-#include <string.h>
 #include <unistd.h>
 #include <sys/stat.h>
 #include <sys/wait.h>
@@ -44,25 +43,25 @@ static const char *PROG =
 static int
 run_stage(const char *name, const char *driver)
 {
-	char src[64], tmpdir[64], cmd[2048];
-	snprintf(src, sizeof src, "/tmp/dtl_%d.ww", getpid());
+	char tmpdir[64], src[128], outbin[128], rmcmd[160], cmd[2048];
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/dtl_%d_d", getpid());
+	mkdir(tmpdir, 0755);
+	snprintf(src, sizeof src, "%s/dtl_%d.ww", tmpdir, getpid());
+	snprintf(outbin, sizeof outbin, "%s/dtl_%d", tmpdir, getpid());
+	snprintf(rmcmd, sizeof rmcmd, "rm -rf %s", tmpdir);
 
 	FILE *f = fopen(src, "wb");
-	if (!f) return 1;
+	if (!f) { runwait(rmcmd); return 1; }
 	fputs(PROG, f);
 	fclose(f);
 
-	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, src);
+	snprintf(cmd, sizeof cmd, "%s build -o %s %s 2>/dev/null",
+	    driver, outbin, src);
 	if (runwait(cmd) != 0) {
 		fprintf(stderr, "dirs_toolong[%s]: build failed\n", name);
-		unlink(src); rmdir(tmpdir);
+		runwait(rmcmd);
 		return 1;
 	}
-	char outbin[128];
-	snprintf(outbin, sizeof outbin, "%s/dtl_%d", tmpdir, getpid());
 
 	int fail = 0;
 
@@ -101,7 +100,7 @@ run_stage(const char *name, const char *driver)
 		(void)system(cmd);
 	}
 
-	unlink(src); unlink(outbin); rmdir(tmpdir);
+	runwait(rmcmd);
 	snprintf(cmd, sizeof cmd, "rm -rf /tmp/dtl_%d_home", getpid());
 	(void)system(cmd);
 	return fail;