docs: document peer id range (1–64) as architecture constraint

[krwg]

Summary

normalizePeerIdList and handshake verification enforce peer ids 1..64. Find whether this is a structural protocol/data constraint (e.g. bitmask) or a soft product limit, then document it next to trust/peer validation so larger group UIs do not discover it late.

Work

• Trace origins of 64 in main/renderer peer paths
• Document constraint + implications for group size / call roster
• Only propose raising the limit if the data structure allows it (separate issue if so)

Acceptance

• Constraint documented (code comment + short architecture note)
• Call sites that enforce 1–64 referenced from that note