FLEX-WP-0036: binary renamed to access-engine; flex-auth is a stub naming the rename (FLEX-DEC-2026-020)

- cmd/flex-auth -> cmd/access-engine; usage and serving lines say access-engine
- cmd/flex-auth: prints the rename and exits 2, with a test
- image: entrypoint /usr/local/bin/access-engine, flex-auth stub alongside;
  live pins pass only args, so their next roll is unaffected
- Makefile, CI, verify-posture, tenancy.yaml, live docs updated; workplans,
  decisions and ADRs left as written
- runtime names (module, audience, k8s, image package, /opt paths) unchanged

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 735967@bnt-lap001
Assistant-Session: 5971967f-77f8-4ced-9e02-c6ef67040ffc

tools/verify-posture.sh

@@ -3,7 +3,7 @@ set -euo pipefail
 
 root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
 declaration="$root/tenancy.yaml"
-main="$root/cmd/flex-auth/main.go"
+main="$root/cmd/access-engine/main.go"
 
 axis_value() {
   local block="$1" axis="$2"