docs: prefork guide, streaming guide, updated comparison

[pratyush618]

Summary

• Prefork pool guide (docs/guide/prefork.md) — when to use, getting started, architecture diagram, configuration, debugging, migration from thread pool, limitations
• Result streaming guide (docs/guide/streaming.md) — publish/stream API, async streaming, FastAPI SSE, patterns (ETL, ML training, batch processing)
• Updated comparison table — 5 new rows: prefork, namespace isolation, result streaming, worker discovery, lifecycle events
• Nav updated — both new guides added to zensical.toml

Test plan

• Docs are markdown-only, no code changes