Collide as the reference keeps it: encoded body sims on the contact, dense fat boxes (#121)

[nicolas-maman]

Ports two pieces of Box3D e77352c's collide pass that were missing (#121):

• Encoded body sims on the contact (encodedBodySimA/B): the awake index for an awake body, -(index + 2) for a static one, NULL_INDEX otherwise. They are kept current wherever a sim moves, as the reference does: contact creation, entering the graph, wake, sleep, and the sim swapped into a destroyed or transferred body's place (refresh_body_contact_indices, the reference's b3RefreshBodyContactIndices). The collide pass and both contact prepares read them instead of going shape, body, set, sim and rewriting the indices every step. A sleeping body's sim is found through the body (resolve_contact_body_sim, the reference's b3ResolveContactBodySim).
• Dense fat boxes (World.fat_aabbs by shape id, the reference's fatAABBs): the overlap test reads two entries of a dense array, and a recycled contact never touches its shapes.

Nothing is computed differently: the golden determinism trace (1136 steps) and every benchmark checksum are unchanged.

Edge cases: validate_solver_sets now checks every contact's encodings against its bodies (as b3ValidateSolverSets does), so every existing test that validates covers them. A new test_encoded_sims in tests/test_edge_cases.ae drives each path that moves a sim, validating after every step: islands sleeping and waking one at a time, a non-touching contact between an awake body and a sleeping one, a static and an awake sim swapped into a destroyed body's place, kinematic and back, disabled and enabled. Leaving out the refresh on a destroy fails 4 of its checks, and leaving out the wake fixup fails 8.

Pinned to one core, 5 runs each rotated, ms per step (median):

scene	main	this PR	Box3D e77352c, AVX2
large pyramid	6.81	6.47	5.39
collide	1.00-1.08	0.81-0.85	0.81-0.94
many pyramids	14.86	14.34	12.40
collide	2.49-2.97	1.81-1.97	1.97-2.36
joint grid	10.75	10.85	10.27
mixed pile, awake	2.58-2.64	2.52-2.55	1.99-2.02

Collide is level with the reference on the large pyramid and ahead on the many pyramids. The scenes stand at 1.2x, 1.15x and 1.05x; the mixed pile at 1.26x, its collide still behind because more of its contacts compute manifolds afresh in double. The rest of #121 is the lanes (aether#2550), the prepare, and the pairs' double tree.

Local gate on Aether 0.794.0: every suite passes and the golden trace matches bit for bit. Pins: Aether 0.794.0 and aeb v0.325, the latest releases.

Refs #121.