Synthesize per-topic events from event instances with stream-parity tests

[Kane610]

Summary

This PR executes the event-instance parity plan by introducing a synthesis path that converts event instance metadata into initialized Event objects grouped per topic.

The goal is to make event-instance-derived events content-equivalent to stream-derived events on:

• topic
• source
• id
• state
• group

Operation parity is intentionally excluded; synthesized events are initialized by design.

What Changed

1) Model synthesis primitives

• Added Event.synthesize_initialized(...) in axis/models/event.py.
• Added TOPIC_TO_INACTIVE_STATE mapping to normalize inactive defaults for topics with non-numeric states.
• Added event-instance normalization helpers and EventInstance.to_events() in axis/models/event_instance.py:
  • source extraction (including list values expansion)
  • representative data-value extraction (prefers active where present)
  • conversion to initialized Event objects
• Hardened EventInstance.decode() against missing MessageInstance.

2) Interface/lifecycle wiring

• Added EventInstanceHandler.get_events_per_topic() in axis/interfaces/event_instances.py.
• Extended Vapix.initialize_event_instances() to populate
  vapix.event_instance_events: dict[str, list[Event]] in axis/interfaces/vapix.py.

3) Verification tests

• Added parity tests in tests/test_event_instances.py that compare synthesized vs stream events for overlapping topics (PIR, Light Status, VMD4 profile) using the agreed parity fields.
• Added coverage that synthesized output includes unknown/non-enum topics.
• Extended tests/test_vapix.py to verify initialize_event_instances() populates synthesized per-topic events.

Commit Breakdown

1. 7e879e9 Add event-instance to initialized-event synthesis primitives
2. d634b33 Expose synthesized per-topic events from event instances
3. fb5b355 Add parity tests for synthesized event-instance events

Validation

• Targeted tests:
  • uv run pytest tests/test_event_instances.py tests/test_vapix.py -v
• Pre-commit hooks ran successfully on each commit:
  • ruff check
  • ruff format
  • mypy

Notes

• This PR does not change stream-event behavior in EventManager.
• This PR does not merge anything; review is requested first.