README claims "AES-512" encryption, which does not exist #80

Found while specing #46.

Problem

README.md:17 advertises "secure AES-512 encrypting of sensitive data". AES has no 512-bit variant — it supports 128, 192, and 256-bit keys only. The claim is false and misleading about the product's security posture.

Fix

Correct the README line to AES-256 (the real maximum, and what the fixed EncryptionProvider should use after #46's SHA-256 KDF, which yields a 256-bit key).

• Comprehensive Security: Ensure data integrity and confidentiality with robust authentication mechanisms and secure AES-256 encrypting of sensitive data.

Acceptance

• README.md no longer claims AES-512.
• CI gate green.

Related to #46. #46's spec already notes this and will correct the line as part of that PR, so this issue may be closed by #46 — filed for tracking.