Required pieces: `Event` data model, `Observer` Submodule, and `jsonl` Writer

[Amir-R25]

Description

In this work we would like to add:

• observability submodule
• Event datatype:
  • Data model:

    event_id: str
    event_type: str # "llm.call" | "retrieval.query" | "evaluator.metric_result" | "session.start" | "evaluator.aggregate_result" ...
    timestamp: datetime.timestamp
    scope_type: str # "session" | "retrieval" | "evaluation"
    scope_id: str  # "session_id" | "evaluation_id"
    parent_scope_id: str | None
    payload: dict

• Observer class that handles the event stream queue.
• Writer Protocol
  • jsonl Writer
  • Writer exceptions are caught and logged inside the consumer
• Out of scope: publish_event api, contextvar reads, emission sites in scopes, and viz integration (DuckDB)

Requirements

• This module should be independent of the rest of the framework
• Observer should be able to separate queues for different writers.