Review SCM files and security updates (#7)

- Fix security.txt expiry date (was template placeholder)
- Fix META.scm syntax error (cross-platform-status)
- Add SPDX headers to guix.scm
- Improve guix.scm synopsis/description
- Create flake.nix for Nix fallback support (RSR compliance)
- Update STATE.scm with session history
- Add Cargo.lock for reproducible builds

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath

.well-known/security.txt

@@ -2,7 +2,7 @@
 # https://securitytxt.org/
 
 Contact: security@hyperpolymath.org
-Expires: {{EXPIRY_DATE}}
+Expires: 2026-12-17T23:59:59.000Z
 Encryption: https://hyperpolymath.org/gpg/security.asc
 Preferred-Languages: en, nl
 Canonical: https://github.com/hyperpolymath/proof-of-work/.well-known/security.txt