epic: SSE — server-side encoding, tooling, and docs #240

Context

sse is a 708-line, three-layer module (transport-agnostic parser, HTTP client, convenience functions) with deps = ["httpclient"]. It is client-only: consuming text/event-stream works, but producing SSE from httpserver still means hand-writing data: / event: / id: / retry: lines and the text/event-stream header.

Decision: keep sse separate from httpclient. The parser is transport-agnostic, reconnection and Last-Event-ID are protocol logic, and httpclient should stay a transport. The dependency direction (sse → httpclient) is already correct.

Workstreams

• Server-side event encoding: encode_event, validation, tests, docs (see sub-issue)
• Tooling and docs: make test-sse / make benchmark-sse, httpclient/httpserver cross-links (see sub-issue)

Non-goals

• Merging sse into httpclient.
• SSE-specific HTTP/2 work; multiplexed streaming follows feat(http2): HTTP/2 module design and integration with httpclient + httpserver #239.

Acceptance criteria

• sse can format events for httpserver without importing it.
• make test-sse / make benchmark-sse exist and are in the aggregate targets.
• Docs (EN/ZH) cover client + server usage and cross-link from httpclient/httpserver.
• Encoder round-trips through EventSource.
• Version bump (0.4.0), manifest.json, changelog.