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/950_selfcheck.c

@@ -1,10 +1,7 @@
 /*
- * 950_selfcheck — selfhost typechecker emits the same errors the
- * C-side checker does for tagged-union misuse.
- *
- * Each row is a .ww fixture + an expected stderr substring. We run
- * `wwdump_ww -r` (which invokes the selfhost checker's resolve +
- * typed passes) and grep for the substring.
+ * Parity pin: the selfhost typechecker must emit the same errors the
+ * C-side checker does for tagged-union misuse. `wwdump_ww -r` is the
+ * probe — it invokes the selfhost checker's resolve + typed passes.
  */
 #include <stdio.h>
 #include <stdlib.h>
@@ -75,7 +72,6 @@ writefile(const char *path, const char *src)
 struct row { const char *src; const char *expect; };
 
 static const struct row rows[] = {
-	/* match: missing variant */
 	{ "fn pick() (i32 | str | bool) = { return 1; };\n"
 	  "fn caller() void = {\n"
 	  "  let v: (i32 | str | bool) = pick();\n"
@@ -85,14 +81,12 @@ static const struct row rows[] = {
 	  "  };\n"
 	  "};\n",
 	  "match: variant not handled" },
-	/* ?: enclosing not tagged */
 	{ "fn inner() (i32 | str) = { return 1; };\n"
 	  "fn caller() i32 = {\n"
 	  "  let v: i32 = inner()?;\n"
 	  "  return v;\n"
 	  "};\n",
 	  "?: enclosing fn return is not tagged" },
-	/* ?: error variant not in enclosing return */
 	{ "fn inner() (i32 | str | bool) = { return 1; };\n"
 	  "fn caller() (i32 | str) = {\n"
 	  "  let v: i32 = inner()?;\n"
@@ -117,7 +111,6 @@ static const struct row rows[] = {
 	  "  return v + 1;\n"
 	  "};\n",
 	  "?: error variant not in enclosing return" },
-	/* case T => names a non-variant */
 	{ "fn pick() (i32 | str) = { return 1; };\n"
 	  "fn caller() void = {\n"
 	  "  let v: (i32 | str) = pick();\n"
@@ -128,19 +121,16 @@ static const struct row rows[] = {
 	  "  };\n"
 	  "};\n",
 	  "case: not a variant" },
-	/* is T where T isn't a variant of the operand */
 	{ "fn pick() (i32 | str) = { return 1; };\n"
 	  "fn caller() void = {\n"
 	  "  let v: (i32 | str) = pick();\n"
 	  "  if (v is f64) { };\n"
 	  "};\n",
 	  "is/as: not a variant" },
-	/* let init-type mismatch on primitives */
 	{ "fn caller() void = {\n"
 	  "  let x: bool = 42;\n"
 	  "};\n",
 	  "let: not assignable" },
-	/* return type mismatch */
 	{ "fn caller() i32 = {\n"
 	  "  return \"hi\";\n"
 	  "};\n",