fix(rest): bank the 2-site siblingCode ratchet drop from #7981 (77 -> 75)

registerSecurityEndpoints' conversion onto the shared respondError helper
moved 2 write sites off the { code, error } sibling-code dialect. The
route-envelope conformance gate (#3843) treats a lower found-count as
progress to bank, not a pass: lower the declared ceiling in MODULES so the
ground cannot be silently given back.

Measured: merge-base (e3c8ed0) siblingCode=77, branch head siblingCode=75.
The 2 vanished sites are merge-base lines 9529/9532, both inside
registerSecurityEndpoints -- exactly the function PR #8071 converted.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B3Kurx8qufrDzNjk4rag7V

Author: claude

scripts/check-route-envelope.mjs

@@ -244,7 +244,10 @@ const MODULES = {
       'the repo\'s hottest response file (208 write sites, edited several times a day) — a `responses` ratchet would go red for edits that are not envelope drift',
     ratchet: '#7035 (option 1: convert onto the shared sendOk/sendError)',
     stringError: 44,
-    siblingCode: 77,
+    // 77 → 75 (#7981): registerSecurityEndpoints' two `handleError` arms moved
+    // off the `{ code, error }` sibling-code literal onto the shared
+    // `respondError` helper, banking that progress per the ratchet's own rule.
+    siblingCode: 75,
   },
 };