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/testenv/testenv.ww

@@ -1,10 +1,8 @@
 package testenv;
 
-// Shared plumbing for ww-native test drivers under test/: subprocess
-// launch with captured output, file IO, string search, and scratch
-// ownership. A utility surface lifted from the proven
-// test/package/package_test.ww idiom — not a framework: callers own
-// discovery, assertions, and verdicts.
+// A utility surface lifted from the proven test/package/package_test.ww
+// idiom — not a framework: callers own discovery, assertions, and
+// verdicts.
 
 import endian;
 import os;