Add Event class as foundation for deployables and multistage simulation

[Copilot]

Pull request type

• Code changes (bugfix, features)
• Code maintenance (refactoring, formatting, tests)

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• All tests (pytest tests -m slow --runslow) have passed locally

Current behavior

Flight events (parachute deployments) are stored as raw [time, parachute] two-element lists in Flight.parachute_events. There is no structured type to represent discrete simulation events, making it impossible to cleanly extend the system for stage separations, deployable releases, or other future event types.

New behavior

Introduces Event — a lightweight class in rocketpy/simulation/event.py — to represent any discrete occurrence during a flight simulation.

Why simulation/, not rocket/?
rocket/ defines vehicle geometry and components (what exists). simulation/ records what happens during flight (when and how components activate). Event is created by the ODE solver inside Flight; it belongs where it's used. This also keeps it reusable for future MonteCarlo and multistage managers without circular imports.

Key attributes:

Attribute	Description
time	Trigger time in seconds
trigger	The triggering condition (mirrors Parachute.trigger)
event_type	"parachute", "stage_separation", "deployable", etc.
action	The associated object (e.g., the Parachute that deployed)

Usage:

from rocketpy import Event

# After a flight simulation, parachute_events holds Event objects
for ev in flight.parachute_events:
    print(ev.time, ev.event_type, ev.action.name)

# Backward-compatible: existing index access still works
t = flight.parachute_events[0][0]   # ev.time
p = flight.parachute_events[0][1]   # ev.action

Changes:

• rocketpy/simulation/event.py — new Event class with __repr__, __str__, __eq__, and backward-compatible __getitem__
• Flight.__simulate() — both parachute trigger paths now append Event objects instead of raw lists
• flight_prints.py — updated to use event.time / event.action attributes
• rocketpy/simulation/__init__.py, rocketpy/__init__.py — Event is publicly exported
• tests/unit/simulation/test_event.py — 24 unit tests

Breaking change

• No

Existing code accessing parachute_events[i][0] (time) or parachute_events[i][1] (parachute) continues to work via Event.__getitem__.

Additional information

This is the first building block toward the deployables and multistage plan. The event_type string field is intentionally open-ended so future separation events ("stage_separation", "deployable") slot in without changing the class interface.

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