test: re-cite retired-mechanism comments in surviving carriers

The migrate-and-retire audit left the surviving carriers as the only
files still describing the retired combined.ww amalgamator as live:
m2wwi/m3sep headers claimed .wwi was dead code, four byteid carriers
carried dead cleanup of never-written .combined.ww intermediates, and
sepbuild's enumeration mirror plus lib_byteid's completeness scan
still skipped the retired artifact form (the same silent
accommodation just removed from both drivers — dropping it here
surfaced and flushed 39 stale untracked amalgamator outputs across
lib/, which the gates now reject loudly). Remaining mentions are
past-tense history or quoted diagnostics.

test/wcc/995_self_rebuild.c

@@ -88,7 +88,7 @@ struct buildjob {
  * inherits no concurrent siblings — system() spawns a fresh /bin/sh -c.
  * stderr lands in <workdir>/build.err so concurrent builds don't merge
  * their diagnostics. `-o <workdir>/main` (T3) makes the driver write its
- * intermediates (main.{combined.ww,s,o}) beside the output in <workdir>
+ * intermediates (main.{s,o} and the sepwork tree) beside the output in <workdir>
  * instead of next to every traversed source — so concurrent driver
  * builds no longer share the next-to-source fixed paths, and this gate
  * may run in the parallel group. The rebuilt binary still lands at