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/989_modresetadj_run.c

@@ -33,8 +33,7 @@
  * assembled+linked program runs to its return value (the emitted code is
  * valid, not just parse-clean).
  *
- * Light wwstage-driver test (CLAUDE.md rule 14): every intermediate is
- * `-o`-redirected to /tmp, so it is phase-1 parallel-safe and never lands
+ * Every intermediate is `-o`-redirected to /tmp and never lands
  * next to a source. Models 989_declns_sep + 994_w6c_ww conventions; 989
  * prefix per the sep-gate precedent (the 7xx range is exhausted).
  */