toolchain: banner purge + WHY-only comment sweep (rule 8)

selfhost/, cmd/, internal/ join the tree-wide sweep: every section
banner dies (91 selfhost + the cmd C-style dividers -> 0); narration
and stale contracts deleted (pre-#22 bundler notes, retired
single-PT_LOAD and no-archive claims, superseded ABI tables); every
ref/harec/qbe cite, task cite, encoding/ELF contract, and rule-10
twin pointer kept; lost lifetime/rationale lines restored where the
sweep over-cut (elf_globals ownership, kwtab linear-scan). Comment-
only proven: all five wwstage tool binaries byte-identical across
the sweep; test-commit, test-byteid (161+1399, 0 pinned-divergent),
and test-bootstrap (fixed point + 991-995 byte-id) all exit 0.
The read-through banked 66 latent-bug leads (checkpoint).

cmd/w6c/gc.h

@@ -1,7 +1,4 @@
-/*
- * gc.h — w6c-private header: Prog/Adr structs, scratch register set,
- * stack-frame state. Plan 9 cmd/6c/gc.h shape, trimmed.
- */
+/* Plan 9 cmd/6c/gc.h shape, trimmed. */
 #ifndef SIX_GC_H
 #define SIX_GC_H
 
@@ -73,9 +70,7 @@ void	emit(Cg*, Prog*);
 /* txt.c */
 void	txt_emit(FILE*, Prog *head);
 
-/* wwi.c — `.wwi` export-data producer (w6c -I). M2 dead-code: writes a
- * re-parseable ww-prototype rendering of the package's exported surface.
- * Returns non-zero if check_exported_type rejects a dangling export. */
+/* wwi.c — non-zero return: check_exported_type rejected a dangling export. */
 int	wwi_emit(Checker *c, FILE *of, Node *file);
 
 /* swt.c, peep.c, reg.c — placeholders for now */