feat(rpc): add Dogecoin bench #235

[Flotapponnier]

Summary

• New bench hl-frontends: switch headline from rolling 24h to last complete UTC day #235: Dogecoin RPC latency (Tatum, dRPC, BlockCypher)
• probe.go: adds dogecoin kind with URL-based dispatch — JSON-RPC POST for Tatum/dRPC, REST GET for BlockCypher
• config.go: dogecoin chain entry with 3 keyless providers
• benchmarks/dogecoin-rpc.yml: full bench spec with per-region queries
• brand.ts / logo-manifest.ts / provider-registry.ts: blockcypher entry
• public/logos/blockcypher.svg: BlockCypher logo

Test plan

• Go build passes (go build ./... in harnesses/rpc-capabilities — verified locally)
• Merge → trigger Railway redeploy on all 3 rpc-capabilities services (us/eu/sgp)
• Wait ~5 min, verify rpc_latency_milliseconds{chain="dogecoin"} appears in Prometheus
• Rebuild materialize-worker on VPS to pick up new bench YAML
• Check /benchmarks/dogecoin-rpc loads with live data

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