feat!: Return a list instead of a set from the collision detection methods

[spydon]

Description

The collision detection system previously passed the intersection points around as a
Set<Vector2>, which meant that every collision check in the hot loop had to hash
Vector2s and allocate hash sets. This PR changes all of these APIs to use
List<Vector2> instead.

The returned points are still guaranteed to be unique: the polygon and circle-polygon
intersection accumulation now deduplicates with a linear contains check (the point
lists are tiny, so this is cheaper than hashing), and tangent circles now explicitly
return a single point instead of relying on the set to collapse the two identical
solutions.

Changed APIs (all from Set<Vector2> to List<Vector2>):

• CollisionCallbacks.onCollision/onCollisionStart and the corresponding methods on
  GenericCollisionCallbacks, ShapeHitbox and CollisionPassthrough, as well as the
  CollisionCallback typedef.
• CollisionDetection.intersections, handleCollisionStart and handleCollision
  (and the StandardCollisionDetection overrides).
• Hitbox.intersections and ShapeHitbox.intersections.
• Intersections.intersect/unorderedIntersect and the top-level intersections
  function in the geometry package.
• The experimental Rectangle.intersections.

Benchmarked the accumulation strategies against each other with the real intersection
workloads (200k iterations per case): the list version is roughly 2x faster for
circle-circle intersections, faster for overlapping axis-aligned rectangles (which
produce collinear duplicate candidates), and equal within noise for tilted polygons.

flame_behaviors and all examples, docs and tutorials have been updated accordingly.

Migration: change the Set<Vector2> intersectionPoints parameter type to
List<Vector2> in onCollision/onCollisionStart overrides (and in custom
CollisionDetection/Hitbox implementations).

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Migration instructions

Replace Set<Vector2> with List<Vector2> in your onCollision and onCollisionStart
overrides and collision callbacks:

@override
void onCollision(List<Vector2> intersectionPoints, PositionComponent other) {
  // ...
}

The same applies to custom CollisionDetection, Hitbox and Intersections
implementations, which now return List<Vector2> from their intersection methods.
The points in the list are still unique.

Related Issues

Closes #4001

[luanpotter]

I am not happy, but I accept