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

Every section banner dies (103 -> 0) across test/lang, the observer
suites, the C carriers, and the five comment-heavy corpus fixtures;
banner provenance (#N cites, carrier numbers, repair-cluster labels)
folded into headers or adjacent WHY comments. Narration deleted; row
provenance, ref cites, divergence pins, and layout contracts kept
(fwd-ref decl-order guards and bootstrap-gate corpus rationale
restored where the sweep over-cut). Comment-only proven: all 3742
wwbuild workdir .s byte-identical before/after; test-commit and
test-byteid (161 lang + 1399 data, 0 pinned-divergent) green.

test/wcc/data/selfhost_smoke/case.ww

@@ -24,8 +24,6 @@ import os;
 import strconv;
 import ascii;
 
-// --- bump arena ---------------------------------------------------------
-
 type arena = struct {
 	buf: *u8,
 	off: u64,
@@ -49,18 +47,14 @@ fn arena_alloc(a: *arena, n: u64) *u8 = {
 	return p;
 };
 
-// --- (i32 | str) error idiom -------------------------------------------
-
 fn checked_div(num: i32, den: i32) (i32 | str) = {
 	if (den == 0) { return "div by zero"; };
 	return num / den;
 };
 
-// --- struct-of-fn-pointer polymorphism ---------------------------------
-//
-// A trivial "writer" abstraction: a function pointer plus a context.
-// This mirrors how io.stream / Plan 9 Bio work. The ctx pointer lets
-// the implementation own its own state without a global.
+// A "writer" is a function pointer plus a context, mirroring how
+// io.stream / Plan 9 Bio work. The ctx pointer lets the implementation
+// own its own state without a global.
 
 type counter = struct {
 	n: i32,
@@ -76,22 +70,19 @@ fn count_emit(ctx: *void, b: u8) void = {
 	c.n += 1;
 };
 
-// --- size/align/offset typed-builtin fixtures (#42) --------------------
-
 type point = struct {
 	x: i32,
 	y: i32,
 };
 
-// Mixed-alignment struct: i8 lays at 0, then i64 needs to skip to
-// offset 8 (the i64's natural align). Probe asserts both ends.
+// #42 size/align/offset typed-builtin fixture. Mixed-alignment struct:
+// i8 lays at 0, then i64 needs to skip to offset 8 (the i64's natural
+// align). Probe asserts both ends.
 type mixalign = struct {
 	tag: i8,
 	val: i64,
 };
 
-// --- byte scanner like lex.ww's hot path -------------------------------
-
 fn count_digits(s: str) i32 = {
 	let i: i32 = 0;
 	let n: i32 = 0;
@@ -105,8 +96,6 @@ fn count_digits(s: str) i32 = {
 	return n;
 };
 
-// --- entry --------------------------------------------------------------
-
 export fn main() i32 = {
 	// Probe 1 — arena hands out distinct pointers, refuses oversize.
 	let buf: [256]u8;