Small SEP-aligned groundwork branch (unable to open PR)

[Vishwas Bajaj]

Hi Vincent,

I’ve been closely following the SEP discussion around the new public-key
cryptography framework, especially the design-first approach and the
emphasis on getting the structure right before adding concrete schemes.

To ground that discussion a bit, I put together a very small,
self-contained branch that adds only documentation and test scaffolding
(no APIs and no cryptographic logic). The idea is to make the intended
invariants and testing direction explicit early, so future
implementations can plug in cleanly and be validated via TestSuite.

I’m currently unable to open pull requests on GitHub due to account
restrictions, but if this looks useful I’d be happy to rebase, provide
patches, or have someone open the PR on my behalf.

Branch link:
https://github.com/v02709928-png/sage/tree/pk-crypto-test-scaffold

Best regards,
Vishwas