Physics: add triggers/sensors (non-solid overlap colliders)

[stormmuller]

Problem

Every collider in src/physics is solid — there's no way to detect overlap between two shapes without the collision-resolution system also applying an impulse to separate them. Trigger volumes (pickups, checkpoints, damage zones, area-of-effect regions) are a standard building block missing from the engine.

Impact

Any game needing "detect overlap but don't physically resolve it" behavior currently has to hack it (e.g. by manually checking AABB/shape overlap outside the physics module, duplicating narrow-phase logic), or can't build it cleanly at all.

Suggested fix

• Add an isTrigger (or similarly named) flag to collider components.
• Narrow phase should still detect overlaps for trigger colliders, but the collision-resolution system should skip impulse/penetration-correction for any contact involving a trigger.
• Emit enter/stay/exit overlap events (or expose queryable overlap state) so game code can react, consistent with the engine's existing event system (ForgeEvent).

Acceptance criteria

• A collider can be marked as a trigger and produces overlap detection without physical resolution
• Trigger-vs-solid and trigger-vs-trigger behave correctly (no impulses applied)
• Enter/exit overlap is observable by game code
• Covered by physics unit tests
• Documented in documentation-site/docs/docs/physics