feat: add Backend::latest_best_block_ref and OnlineClient::at_current_best_block

[ilchu]

A one-shot accessor for the current best block, next to the finalized one. Defaults stay on finalized; this is an opt-in for callers that want the newest view and accept that it can be reorged or pruned.

Motivation

#1126 added Backend::latest_best_block_ref; #1161 moved every default to the finalized block because the best block "might actually be pruned in the near future" (#1161 (comment)) and dropped the method as unused, noting a user "can still provide a block hash that they prefer" (#1161 (comment)). There is no direct way to get that hash with a pin from a chainHead-backed client: stream_best_blocks() replays the finalized hashes, buffered NewBlocks and the latest BestBlockChanged with no end marker, each item behind a chainHead_v1_header round trip (the integration tests use a 200 ms timer, consume_initial_blocks, for the same problem on the finalized stream); LegacyRpcMethods::chain_get_block_hash(None) is legacy-only and unpinned.

Use case: a server authorising requests against chain state wants to read at its newest block instead of making a client that already saw finality wait for the server's (paritytech/web3-storage#384).

Changes

• Backend::latest_best_block_ref; the trait is sealed, all four in-tree backends implement it.
• chainHead: FollowStreamDriverSubscription::latest_best_block awaits Initialized, drains the already-queued events with now_or_never, returns the last BestBlockChanged else the last finalized hash. A queued Stop restarts the wait (those blocks are unpinned). The BlockRef keeps the block pinned like latest_finalized_block_ref. Right after a (re)subscription, before the node reports a best block, it returns the finalized head.
• legacy: chain_getBlockHash(None) in retry(), as in Introduce Backend trait to allow different RPC (or other) backends to be implemented #1126. archive: error. combined: chainHead, then legacy.
• OnlineClient::at_current_best_block(), plus a mention in the introduction docs.

No existing behaviour changes.

Tests

follow_stream_driver.rs: replayed BestBlockChanged wins over the finalized head; empty replay falls back to it; Stop in the replay restarts and returns the new finalized head (fails without the restart). Integration: best_block_is_at_or_ahead_of_finalized_block (full client, all backend/rpc combinations) and best_block_lookup (light client).