feat(sse): server-side event encoding #241

What

Add server-side formatting to sse (currently client-only).

• encode_event(data="", *, event=None, id=None, retry=None) -> str — one complete SSE block, terminated by a blank line.
  • Multi-line data becomes one data: line per line; CRLF and lone CR are normalized to LF.
  • Empty data still emits data: so the block dispatches.
  • id must not contain NUL; retry must be a non-negative integer.
• A comment/heartbeat helper (encode_comment) if it can be made unambiguous.
• Optional SSEEvent.encode() convenience that delegates to encode_event.
• Unit tests: field formatting, multi-line data, empty data, validation errors, round-trip through EventSource.
• Docs (EN/ZH): a server-side example using httpserver.StreamingResponse with content_type="text/event-stream".
• Bump sse 0.3.4 → 0.4.0; refresh manifest.json.

Why

httpserver has StreamingResponse but no SSE formatter. Users hand-roll the wire format and get the details wrong: multi-line data:, exactly one leading space after the colon, the blank line that dispatches, NUL in id, non-integer retry.

Acceptance

• encode_event matches the WHATWG field syntax and round-trips through EventSource.
• The encoder path imports neither httpserver nor httpclient (pure stdlib).
• Tests and docs pass; manifest and version updated.