security: side-channel safety & constant-time operations

[lonerapier]

Guarantee constant-time arithmetic on secret values to prevent timing attacks.

Approach

• Integrate the subtle crate for constant-time comparisons and conditionals.
• Audit all field and share operations for secret-dependent branches.
• Add ct_eq and Choice where needed.

Tasks

• Integrate subtle crate.
• Replace unsafe branches with subtle conditionals.
• Add timing tests or analysis harness.
• Update SECURITY.md.