feat: add 'manual' SSE mode for runtime stream/non-stream decisions

[CatchMe2]

What

Adds a new sse: 'manual' route mode (shorthand sse: 'manual' and object form { kind: 'manual' }) that lets a handler decide at runtime whether to stream, without any Accept-header negotiation.

Why

OpenAI-compatible and other LLM APIs decide whether to stream based on the request body ({ "stream": true }) rather than the Accept header — the client sends no Accept: text/event-stream either way.

Behavior

• 'manual' skips Accept negotiation: reply.sse is always attached.
• The handler streams (reply.sse.send(...)) or returns a plain value. SSE headers commit lazily on the first send()/stream(), so a non-streaming return falls through to Fastify's normal JSON serialization uncorrupted.
• Heartbeat start is deferred until headers are committed, so a non-streaming fallback response is never corrupted by a heartbeat write (a correctness improvement that applies to all modes).

Resolves #16

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm