Clarify ConstantStepSize finite-interval step semantics

[Whning0513]

Fixes #757.

This updates the finite-interval documentation for ConstantStepSize and diffeqsolve(dt0=...) to match the current implementation:

• dt0 determines the number of fixed steps via ceil((t1 - t0) / dt0)
• the remaining fixed steps are rescaled so the solve lands exactly on t1

It also adds a regression test that locks in the current step locations for a representative finite-interval solve, plus a docstring test for the clarified wording.

Tested with:

• uv run python -m pytest -q test/test_saveat_solution.py

[Copilot]

Pull request overview

This PR addresses #757 by aligning Diffrax’s documentation with the current ConstantStepSize behavior on finite intervals, and adds tests to lock in both the observed step locations and the clarified wording.

Changes:

• Updates ConstantStepSize and diffeqsolve(dt0=...) documentation to describe finite-interval step placement that lands exactly on t1.
• Updates the getting-started guide to describe ConstantStepSize as fixed-step behavior on finite intervals.
• Adds a regression test asserting the step timestamps for a representative finite-interval ConstantStepSize solve, plus a docstring-content test.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
test/test_saveat_solution.py	Adds a regression test for finite-interval step locations and a docstring wording test.
docs/usage/getting-started.md	Adjusts tutorial wording to describe finite-interval fixed-step semantics.
diffrax/_step_size_controller/constant.py	Updates ConstantStepSize docstring to describe finite-interval behavior.
diffrax/_integrate.py	Updates diffeqsolve’s dt0 docstring text to match finite-interval fixed-step behavior.

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[Whning0513]

Addressed the four review comments in aad4faa. The API docstrings now give the exact target formula, the getting-started guide calls out the special first step, and the regression test computes num_steps with the controller formula instead of hard-coding 11. The two focused tests pass; prek also passes mixed-line-ending, ruff-format, ruff, and pyright checks on the four changed files