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single-PT_LOAD and no-archive claims, superseded ABI tables); every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/swt.c

@@ -1,7 +1,7 @@
 /*
- * swt.c — switch-statement lowering. Stub for now: cgen falls
- * through to a no-op for N_SWITCH. When we add a real lowering, it
- * will live here, mirroring Plan 9 6c's pswt.c.
+ * Stub for now: cgen falls through to a no-op for N_SWITCH. When we
+ * add a real lowering, it will live here, mirroring Plan 9 6c's
+ * pswt.c.
  */
 #include "gc.h"