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single-PT_LOAD and no-archive claims, superseded ABI tables); every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

@@ -1,8 +1,8 @@
 /*
- * reg.c — register allocator. The current cgen pins everything to AX
- * with BX as a scratch top-of-stack — no real allocation. This file
- * is the seam where a linear-scan or graph-colouring pass would land
- * later; today it's empty.
+ * The current cgen pins everything to AX with BX as a scratch
+ * top-of-stack — no real allocation. This file is the seam where a
+ * linear-scan or graph-colouring pass would land later; today it's
+ * empty.
  */
 #include "gc.h"