SSE transport for MEV-Share event streaming

[koko1123]

Motivation

MEV-Share's event stream uses Server-Sent Events (SSE), not WebSocket. eth.zig currently has ws_transport.zig and http_transport.zig but no SSE transport. This blocks a full MEV-Share client.

SSE is also used by other Ethereum tooling (some RPC providers, block explorers), so this is a generally useful transport.

Specification

New file: src/sse_transport.zig

pub const SseTransport = struct {
    /// Open a persistent HTTP connection and parse the SSE event stream.
    /// Calls `callback` for each complete event.
    pub fn subscribe(
        allocator: std.mem.Allocator,
        url: []const u8,
        headers: []const std.http.Header,
        callback: *const fn (event: SseEvent) void,
    ) !void

    pub const SseEvent = struct {
        id: ?[]const u8,
        event: ?[]const u8,  // event type ("tx", "bundle", etc.)
        data: []const u8,    // JSON payload
    };
};

Wire format

SSE is a text protocol over a persistent HTTP/1.1 connection:

data: {"hash":"0xabc...","logs":[...]}\n
\n
id: 42\n
event: bundle\n
data: {"hash":"0xdef..."}\n
\n

Parser needs to handle:

• Multi-line data: fields (concatenate with \n)
• id: and event: fields
• Blank line as event delimiter
• Reconnect on connection drop (respect retry: field)
• Last-Event-ID header on reconnect

MEV-Share event stream URL

https://mev-share.flashbots.net/api/v1/events (mainnet)

Auth header: X-Flashbots-Signature: 0x<address>:0x<sig> (same scheme as flashbots.zig).

Relationship

This is a prerequisite for mev_share.zig client (event streaming) — see companion issue.