Polkadot RPC endpoints: free public URLs by latency

3 endpoints are probed on this chain: Parity's official `rpc.polkadot.io`, OnFinality's public gateway and PublicNode (operated by Allnodes). Every listed endpoint was live-verified with four consecutive `chain_getHeader` probes returning a parsable hex block number before inclusion. Excluded by the audit: 1RPC filters `chain_getHeader` as "Not Allowed" on its privacy relay, Dwellir returned 503 Service Unavailable during the sweep (will be revisited once stable), Ankr requires a paid Polkadot key, Chainstack is key-gated on Substrate, and RadiumBlock rejects requests without a referral header.

One identical Substrate JSON-RPC POST (`chain_getHeader`) every 60 seconds against each provider from each of 3 regions, using the same plain HTTP client the EVM cluster uses. The response is a header object containing a hex-encoded relay-block number and the parent-block hash; the harness parses the number (`strconv.ParseUint(strings.TrimPrefix(hdr.Number, "0x"), 16, 64)`, same rule as EVM). Wall-clock round-trip is recorded at millisecond precision; p50/p90/p99 are computed via Prometheus `quantile_over_time` over 24 hours. Responses are classified (`ok` / `http_err` / `jsonrpc_err` / `stale` / `timeout`) so an endpoint stuck on an old head or returning errors behind HTTP 200 is never ranked as fastest.

Two reasons. First, `chain_getHeader` returns the head block number, which lets the harness plug into the cluster's cross-provider tip machinery for staleness classification without adding a chain-specific code path. Second, `system_health` returns a small object about peer count and sync state that any polite CDN can synthesize without touching the underlying node, defeating the point of a latency probe on the RPC path. `chain_getHeader` forces the endpoint to actually read the head from the runtime, which is what integrators care about.

Polkadot's state model accesses historical state via `state_getStorage` keyed by a Blake2-hashed storage key, which has no chain-agnostic depth analog for the EVM `eth_getBalance(address, height)` probe the cluster uses on other chains. Similarly, `chain_getHeader` returns the parent-block hash (not the current-block hash: the current hash is derived by Blake2 over the SCALE-encoded header), so the cross-provider height-hash quorum check from bench 083 cannot be run against Polkadot without adding a Substrate-native encoder to the harness. Both columns are omitted honestly rather than filled with a proxy that would misrepresent the chain.