Add inertial-physics regression tests for single-cell fall, directional streaming, walls, and τ relaxation

[Copilot]

• Review new PR comment and identify requested action
• Inspect current branch status and recent commits
• Run black/flake8 on files touched by recent inertia-test changes
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Original prompt

write test cases for the inertia model (with inertia:true in a json file). set up a new initial condition with just one single cell with solid, everything else void, and check that it falls at the right rate, with the right velocity. also do directional tests with an initial velocity and no gravity. finally, we haven't implemented anything for interactions with walls or to test the relaxation rate tau, make up some tests for them that test against real physics.

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[benjym]

@copilot flake8/black this and resubmit

[Copilot]

@copilot flake8/black this and resubmit

Ran black --check and flake8 on the updated test files, and both pass as-is. Re-ran targeted tests too (23 passed). The current commit remains valid: 991bbc5.

[Copilot]

Pull request overview

This PR adds comprehensive regression tests for the inertial physics model in the HGD simulation code. The tests verify that the inertia: true configuration is correctly loaded, that particles move in expected directions with given velocities, that wall boundaries properly block particle movement, that tau relaxation follows exponential decay physics, and that gravity-driven particle fall accumulates velocity at the correct rate.

Changes:

• Added JSON config loading test to verify inertia: true can be parsed and applied
• Added parameterized directional streaming tests for zero-gravity transport with initial velocities
• Added wall interaction test to ensure particles respect boundary masks
• Added multi-step tau relaxation test verifying exponential velocity decay
• Added single-particle fall test checking gravity-driven velocity accumulation
• Added <stdexcept> include to core.cpp for exception handling

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/test_stream_lbm_zero_eq.py	Added 4 new tests: JSON config loading, parameterized directional streaming without gravity, wall interaction behavior, and multi-step tau relaxation with exponential decay
test/test_move_particles_core.py	Added single-particle gravity fall test verifying position and velocity changes over multiple time steps
HGD/motion/core.cpp	Added <stdexcept> include required for std::invalid_argument exception used in space_criterion parsing

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