[EPIC] WebSocket & filter RPC

[arner]

Add support for WebSocket & filter RPC.

Why

fxevm's gateway serves standard Ethereum JSON-RPC over plain HTTP only. Two closely related parts
of that standard surface are missing entirely, and both are documented as known, deliberate gaps
rather than oversights (docs/COMPATIBILITY.md:95-96, :464-466):

• WebSocket transport. No ws:// (or wss://) endpoint at all — only HTTP.
• Server-side filters and subscriptions. No eth_newBlockFilter / eth_getFilterChanges /
  eth_uninstallFilter, no eth_subscribe / eth_unsubscribe. The documented workaround is "poll
  eth_blockNumber and eth_getLogs instead."

That workaround is fine for a one-off script, but not for the class of software this gap actually
blocks: anything that needs to react to new blocks or events rather than busy-poll for them —
indexers, monitoring/alerting, relayers, and any Ethereum client library that opens a WS connection
by default instead of falling back to HTTP polling. (Paladin, an EVM privacy-execution engine, is
one concrete example: its core blockchain client refuses to start without a working WS connection,
and its block indexer relies specifically on eth_newBlockFilter/eth_getFilterChanges.)

This epic closes both gaps.

Foundational decision: WS shares the HTTP port

• fxevm serves WS on the same listener/port as HTTP (gateway.Listen), dispatched by the Upgrade: websocket header — exactly how go-ethereum's own node package runs HTTP and WS together (node/rpcstack.go's isWebsocket / httpServer.ServeHTTP).
• This also matches the common Ethereum-node convention of one RPC port serving both transports
  simpler to document, firewall, and deploy than a second port only some callers would ever use.