chore: Add a collision benchmark for Path contour hitboxes

[spydon]

Description

Adds path_collision_benchmark.dart, a harness benchmark that runs the collision detection system with a hitbox built by PolygonHitbox.contour against circles, rectangles, a hand-written polygon, and itself. Each scene has 50 moving, rotating shapes of the subject kind and 50 of the other kind in an 800 by 800 world, and hitboxes only narrow-phase against the partner kind so a scene measures exactly that pairing. The subject is the flame logo path from path_contour_shapes.dart, 37 vertices at 60 pixels. The baseline is the same logo hand-written from its 10 anchor points, which is what a game on main has to use today. The polygon scenes compile on main as well, so the two branches can be compared by running the file on both.

Results on this branch, milliseconds per 20 ticks in JIT through flutter test:

Scene	Path hitbox on this branch	Hand-written polygon on this branch	Hand-written polygon on main
vs circle	17.5	6.7	6.9
vs rectangle	12.4	6.2	6.4
vs 10-vertex polygon	16.0	10.3	10.7
vs itself	92.0	10.3	10.7

Two conclusions from that: the branch does not slow down existing polygons, and the path hitbox pays for its vertex count quadratically against other polygons. #4 fixes most of that in the narrow phase and brings path vs path down to 11.4, and #5 takes it further to 7.7 by making the global vertex and circle computations allocation free.

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.

[adario]

Very cool benchmark!
LGTM