fix(adjoint): keep saves that land on a step end

[Cherith1222]

Why

A recursive checkpoint adjoint drops the derivative of a sample that lands on a step end. Recomputing that step rounds the end two ULPs early, so the save is skipped and the sample adds nothing to the gradient.

What changed

• The save loop keeps a time that sits within two ULPs after the step end on the current step.
• A regression compares this gradient with the direct adjoint when every save lies on a constant step.

Scope

• Covers output times that fall on a step boundary in a checkpointed solve.
• Leaves adaptive step-size control, and saves that sit clearly inside a step, alone.

Blast Radius

This touches solves that record output on step ends and then differentiate them. Forward samples were already correct. The wider test only pulls in a time that rounding had pushed just past the recomputed end.

Verification

• The new test failed before the change, 6.924 against 7.174, and passes after it.
• The oscillator in RecursiveCheckpointAdjoint returns wrong gradients (~2%) when SaveAt(ts) times coincide with ConstantStepSize step ends; result depends on checkpoint count #788 now matches the direct adjoint. Both gradients are 331.67329043219127. The closed form is 331.6732904321968.
• test/test_adjoint.py without the slow test: 11 passed. test/test_saveat_solution.py: 26 passed.

Fixes #788

Drafted with Cursor.