Leios design - Cryptography: refine BLS PoP, aggregation, and benchmarks

[hjeljeli32]

This PR is stacked on top of #599 and refines the cryptography-focused parts of the Leios design document:

• Clarifies BLS Proof-of-Possession construction and DST separation (PoP vs votes).
• Describes aggregation over (election_id, EB_hash) for both persistent and non-persistent voters.
• Adds additional goals to the performance benchmarks section.

[curiecrypt]

LGTM 👍

[ch1bo]

As you expanded on the ## Cryptography section (which I took from the impact analysis), you might want to remove the warning in line 617-621