[Feature] Stronger ordering guarantees for 2D collision events

[vmarcella]

Overview

Define and, if needed, implement stronger ordering guarantees for collision
events so replay systems and deterministic gameplay code can rely on a stable
event sequence.

Current State

The current docs only promise ordering that is stable enough within one run.
Cross-backend and cross-platform deterministic ordering is not guaranteed.

Scope

Goals:

• Decide the required ordering contract for public collision events
• Canonicalize body-pair ordering if deterministic replay is in scope
• Document the performance and implementation cost of stronger guarantees

Non-Goals:

• Lockstep networking implementation
• Backend-specific ordering leaks
• Trigger volumes

Proposed API

No new public API is required if the stronger guarantee can be documented and
implemented within the existing collision_events() contract.

Acceptance Criteria

• Ordering guarantees are documented precisely
• Event collection enforces the documented order when required
• Integration tests cover stable ordering for representative cases
• Any remaining nondeterminism is called out explicitly in docs

Affected Crates

lambda-rs, lambda-rs-platform

Notes

• Sorting by normalized body-pair keys is the most likely implementation
  strategy if stronger ordering is needed.
• This work is critical for replay or rollback systems when they are planned.