Evaluate base64url transport encoding for new envelope format

[michaelmawhinney]

Parent: #19

Problem

Hex is simple and legacy-compatible, but it doubles payload size. A new envelope format could use base64url for more compact JSON, URL, token, and database storage.

Tasks

• Compare hex vs base64url for the new format
• Decide transport encoding
• Keep legacy hex support
• Add malformed encoding tests
• Document the chosen encoding