Harden Nitro parent helper images and add ARM64 validation

[AnthonyRonning]

Summary\n\n- Refresh the two host-side Nitro helpers (credential requester and CloudWatch logger) to a digest-pinned Python 3.13.16 Bookworm base with a verified Linux ARM64 manifest.\n- Replace floating dependencies with complete hash-locked, wheel-only closures; update boto3/botocore and urllib3, remove unused iproute2, and remove pip plus its ensurepip bootstrap after installation.\n- Add offline contract tests and PR CI that builds both Linux ARM64 images, runs the tests without network access, and scans each finished image for fixable high/critical vulnerability matches. Helper-only source changes do not select measured EIF checks.\n\n## Research and validation\n\n- The October 2 parent inventory found existing helper images on Python 3.9 and older urllib3. Python 3.9 is end-of-life; the refreshed boto3/botocore closure supports urllib3 2.8.\n- Both dependency closures installed with wheel-only hash verification, passed pip check and 8 offline contract tests in isolated Python 3.13 environments. pip-audit found no known matches in either lock.\n- Repository pre-commit passed 370 tests; root nix flake check passed on aarch64-darwin.\n- On exact PR head 5565b87, both GitHub ARM64 jobs built their images, ran 5 requester and 3 logger tests inside the images, and passed Trivy v0.75.0 scans with zero fixable HIGH/CRITICAL matches in Debian and installed Python packages. The scanner gate intentionally excludes unfixed and lower-severity matches.\n- Local Docker builds were unavailable because this machine has no working daemon; the CI jobs above supply actual image-build evidence. Synthetic tests still do not establish live VSOCK/IMDS/AWS behavior on a Nitro parent.\n\n## Scope boundary\n\nThis PR does not publish, copy, or deploy images. No dev/prod parent, EIF, PCR approval, KMS policy, or running service is changed. A later rollout requires separate review and live validation. EIF kernel/dependency updates remain a separate follow-up.