fix: enforce cryptography floor in health checks

[CruxExperts]

Summary

• Update architecture documentation to cryptography 50.0.0+.
• Make dependency health and setup checks reject installed cryptography versions below the security floor.
• Preserve valid stable post-release and local-version metadata while rejecting prereleases.
• Add regression coverage for the security floor and version parsing.

This closes the gap where an existing environment with cryptography 49.0.0 could pass health checks after the package floor was raised.

Verification

• pytest tests/test_management.py: 77 passed.
• pytest tests/test_encryption.py: 38 passed.
• Ruff: passed.
• py_compile: passed.
• Version sync and publishing readiness: passed.
• git diff --check: passed.
• Independent review and corrected-delta re-review: no material findings.