[Feature] 2D trigger volumes

[vmarcella]

Overview

Add trigger volumes to the 2D physics API so gameplay code can detect overlap
enter and exit events without generating physical response.

Current State

The collision event system is intentionally limited to solid contacts.

Scope

Goals:

• Add a trigger collider configuration path on Collider2DBuilder
• Emit trigger enter and exit events separately from solid collision events
• Preserve the existing collision event semantics for solid contacts
• Ensure triggers do not participate in solver response

Non-Goals:

• Complex trigger filtering rules beyond layers and masks
• Trigger callbacks during backend simulation
• 3D trigger support

Proposed API

pub struct TriggerEvent {
  pub kind: TriggerEventKind,
  pub body_a: RigidBody2D,
  pub body_b: RigidBody2D,
}

pub enum TriggerEventKind {
  Entered,
  Exited,
}

impl PhysicsWorld2D {
  pub fn trigger_events(&self) -> impl Iterator<Item = TriggerEvent>;
}

impl Collider2DBuilder {
  pub fn with_trigger(self, is_trigger: bool) -> Self;
}

Acceptance Criteria

• Trigger colliders can be configured through the public builder API
• Trigger pairs emit enter and exit events
• Trigger pairs do not produce physical resolution
• Trigger events remain distinct from solid collision events
• Layer and mask filtering applies consistently to triggers
• Integration tests cover trigger enter, steady overlap, and exit
• A demo or tutorial shows trigger usage

Affected Crates

lambda-rs, lambda-rs-platform

Notes

• This should likely map to Rapier sensor colliders internally.
• Trigger semantics should be body-oriented by default for consistency with
  collision events.