test/wcc: carrier ownership repair and driver-contract adaptation

Every surviving carrier now owns its artifacts: checked mkdir/mkdtemp/
fopen acquisition, one all-exit cleanup funnel per carrier, ENOENT-
tolerant checked unlinks, exact-path deletion (rm -rf only for an
owned pid-keyed dir or a .sepwork beneath one), and cleanup failure
fails a passing carrier without overwriting its diagnostic. In the
same pass the carriers adapt to the driver contract this branch lands:
--sep and WW_PKGCACHE are gone, -S and the /tmp/ww_run_<pid> scratch
contract are asserted, and rows whose runtime or reject coverage moved
to test/wcc/data fixtures or test/lang @test owners are trimmed to the
byte/artifact/diagnostic observations only they can make.

Repair and adaptation ride together because most files interleave both
in the same hunks; splitting would manufacture intermediate carrier
states that never existed and cannot run against either driver.

test/wcc/993_ww_ww.c

@@ -1,5 +1,5 @@
 /*
- * 993_ww_ww — phase-10 marker for the ww-side driver port.
+ * 993_ww_ww — explicit bootstrap gate for the ww-side driver port.
  *
  * Diffs the C-built `ww` against the ww-built `ww_ww`. Both drivers
  * orchestrate the same w6c/w6a/w6l, so the resulting binaries must be
@@ -143,8 +143,8 @@ run_exit_code(const char *bin, const char *driver, int code)
 
 /* Stage a program that fails to compile, run it via the named driver,
  * and return the exit code. Pins the build-step caller contract: a
- * failing build must still report failure (non-zero) — the #16 fix that
- * lets procrun return the real exit code must not regress this, since the
+ * failing build must still report failure (non-zero). The process mechanism's
+ * exact exit-code return must not regress this, since the
  * w6c/w6a/w6l callers only test `!= 0`. */
 static int
 run_build_fail(const char *bin, const char *driver)
@@ -268,8 +268,7 @@ main(void)
 	}
 	unlink(hello_src);
 
-	/* #16: `ww_ww run` must propagate the child program's real exit code
-	 * (the shared procrun helper was collapsing every non-zero exit to 1).
+	/* #16: `ww_ww run` must propagate the child program's real exit code.
 	 * Table-driven; the non-zero rows fail under the old collapse-to-1.
 	 * 255 is the single-byte boundary (WEXITSTATUS max). The C ww driver
 	 * is the reference (do_run -> WEXITSTATUS); assert ww_ww matches it