feat(discovery): add zcash.rotko.net to community ZEC servers

[hitchho]

Summary

Adds zcash.rotko.net:443 to the community ZEC server list. Operated by Rotko Networks.

Backend

• zebrad 6.1.0 (state format 28.0.0, Ironwood-ready — NU6.3 activation at block 3,428,143)
• zidecar v0.8.1 fronts the CompactTxStreamer surface on :443 (via haproxy). Optional --zidecar-rpc and --frost-relay surfaces are also enabled but do not conflict with the lwd probe hosh runs.

Verification

$ printf '\x00\x00\x00\x00\x00' | curl -k --http2-prior-knowledge \
    -H 'Content-Type: application/grpc' --data-binary @- \
    https://zcash.rotko.net/cash.z.wallet.sdk.rpc.CompactTxStreamer/GetLightdInfo
# → HTTP 200, protobuf reply carrying:
#   version=0.4.18, vendor=zidecar/rotkonetworks, chainName=main,
#   build=v0.8.1-907a3ed

Chain height reported by the backend matches network tip; verificationprogress = 99.99997%.

AI Disclosure

This PR was drafted with Claude assistance (repository research, diff generation, PR body). All content is human-reviewed.