[WIP] add radiation momentum deposition in photoionization

[BenWibking]

Description

Please write a short paragraph describing what changes this pull request makes to the code. It should provide sufficient context so that a reviewer can efficiently browse the source code changes.

Related issues

Fixes #1988.

Checklist

Before this pull request can be reviewed, all of these tasks should be completed. Denote completed tasks with an x inside the square brackets [ ] in the Markdown source below:

• I have added a description (see above).
• I have added a link to any related issues (if applicable; see above).
• I have read the Contributing Guide.
• I have added tests for any new physics that this PR adds to the code.
• (For quokka-astro org members) I have manually triggered the GPU tests with the magic comment /azp run.

[gemini-code-assist]

Code Review

This pull request introduces a new test case (DTypeFrontVC) and implements the O(v/c) radiation-pressure work term in the photochemistry solver to deposit absorbed photon momentum into the gas. Feedback on these changes highlights two issues: first, a critical bug in photochemistry.hpp where the momentum update is not scaled by energy_update_factor during multi-stage integration (while the kinetic energy subtraction is), which violates energy conservation; second, a potential division-by-zero in the test file if no chem-band flux is absorbed.

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[gemini-code-assist]

If no chem-band flux is absorbed (e.g., due to a setup failure or bug), check.expected_px will be 0.0_rt. This will cause a division-by-zero when computing rel_err and transverse_rel, resulting in NaN or inf values. It is safer to guard these divisions by checking if check.expected_px > 0.0_rt.

	const amrex::Real rel_err = (check.expected_px > 0.0_rt) ? (std::abs(check.gas_px - check.expected_px) / check.expected_px) : std::numeric_limits<amrex::Real>::infinity();
	const amrex::Real transverse_momentum = std::sqrt(check.gas_py * check.gas_py + check.gas_pz * check.gas_pz);
	const amrex::Real transverse_rel = (check.expected_px > 0.0_rt) ? (transverse_momentum / check.expected_px) : std::numeric_limits<amrex::Real>::infinity();

[BenWibking]

/azp run quick

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).