docs: resolve open questions in v2 claim identifier proposal

[collinsezedike]

Summary

Resolves the two open questions from #82 (V2 claim identifier and evidence convention), decided in conversation rather than left open in the doc:

• claim_uri gets a bounded length cap on event payload size (exact byte limit left to the implementation issue).
• The evidence hint on dispute/register is a plain URI, not a hash, evidence is supplementary and informal, not something a voter trusts the way they trust the claim itself.

Status moves from Proposed to Accepted. The implementation issue for this design can now proceed against the doc as written.