chore(gas-estimation): drop Blocknative after API sunset

[Flotapponnier]

Context

Blocknative was acquired by Deloitte and their gas oracle API is in sunset. Every endpoint (api.blocknative.com/gasprices/blockprices, the legacy docs, the gas.network successor) now returns 404 / "fault filter abort". The harness has been counting http_err on every poll for weeks.

What this PR does

• Removes the Blocknative provider entry from benchmarks/gas-estimation.yml, drops it from the abstract, methodology, cadences, findings, FAQ and oracle queries. Oracle count goes from 4 to 3.
• Deletes alternatives/blocknative.yml and the SVG logo. The product no longer exists, no reason to host a comparison page for it.
• Rewrites the Blocknative mentions in alternatives/etherscan.yml and answers/which-gas-oracle-is-the-most-accurate.yml. The "How does Blocknative compare" FAQ is reworded around Etherscan vs PublicNode vs Owlracle.
• Drops the Blocknative scraper from the Go harness: pollBlocknative, the bn* structs, the OracleBlocknative constant, the ?chainid=<N> URL builder, the GAS_TOKEN_BLOCKNATIVE env var and the AuthHeader field on OracleEndpoint (only Blocknative used it).
• Updates src/data/provider-registry.ts, src/components/provider-logo.tsx, src/lib/logo-manifest.ts, src/lib/format.ts, src/app/about/page.tsx.

The remaining 3 providers (Owlracle, Etherscan v2, PublicNode feeHistory) are still working at 95 to 99 percent successRate, so no replacement needed.

Validation

• git grep -i blocknative returns zero matches.
• pnpm typecheck passes.
• go build ./... in harnesses/gas-estimation passes.