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/989_m3sep_run.c

@@ -3,8 +3,8 @@
  *
  * M3 = w6c `-c` (separate-compile / primary-only codegen) reads each imported
  * package's `.wwi` prototypes (M2's producer output) as import scope INSTEAD of
- * re-checking dep bodies. Pure addition: combined.ww stays the LIVE path, `-c`
- * is off on every existing invocation. This gate certifies the consumer.
+ * re-checking dep bodies. Since the sep flip this IS the sole compile
+ * path. This gate certifies the consumer.
  *
  * Fixture: a leaf -> mid -> root chain carrying ALL FOUR cross-boundary fact-
  * classes — fn SIGNATURE (leaf.add), struct/type LAYOUT (leaf.Point, used as a