test: fold driver-parity byteid into 989_lib_byteid, retire 815/940/951

Wwstage-driver-leg byte identity gets one owner. The three carriers'
content identity is already blanket-owned by their r815_/r940_/r951_
corpus twins via test-data-byteid; their only non-redundant assertion
was the ww_ww driver leg, which 989_lib_byteid's 43-unit sweep covers
apart from the zero-dep root-only edge — folded in as a new probe row
(44 units). r940 fixture comments re-cited off the retired carrier.

test/wcc/data/r940_global_sret_pointer/case.ww

@@ -1,5 +1,5 @@
 //ww:run
-// Runtime half of 940_global_sret_run.c global_through_pointer.
+// #220 global-sret family: global mutated through a pointer after the sret assign.
 package main;
 type quad = struct { a: i64, b: i64, c: i64, d: i64 };
 fn mk(x: i64) quad = {