feat(core): support dynamic dt by reading from scene config live

[MuGdxy]

Summary

Replace all cached Float dt members across ~25 CUDA backend systems with live attribute slot references (S<const geometry::AttributeSlot<Float>> dt_attr), enabling users to change the simulation timestep at runtime via scene.config().find<Float>("dt").

Previously, each system copied dt from the scene config once during do_build() or init(), making it impossible to change dt dynamically. Now every system reads dt from the scene config attribute on each use, following the same pattern already established by SimEngine for other config values like m_newton_velocity_tol.

Changes

• SceneVisitor: Add dt() convenience accessor delegating to internal::Scene::dt()
• 25 backend systems: Replace Float dt member with S<const geometry::AttributeSlot<Float>> dt_attr in Impl structs (time integrator, linear subsystems, line search, contact system, active set, animators, constitution managers, diff reporters)
• Tolerance checkers: MaxTranslationChecker and ABDToleranceChecker now recompute abs_tol = factor * dt dynamically in do_check() instead of caching it in do_build()
• Assertions: UIPC_ASSERT after every dt_attr assignment to verify the attribute exists
• Performance: Cache dt_attr->view()[0] into a local Float before entering loops
• Tests: Unit test verifying config read-back via SceneVisitor::dt(), and sim_case test (93_dynamic_dt) verifying end-to-end that changing dt at runtime affects simulation behavior

Fixes #451

[gemini-code-assist]

Code Review

This pull request implements support for dynamic time steps (dt) by refactoring backend systems to access the dt value directly from the scene configuration's attribute slot at runtime. Previously, many systems cached a static copy of dt during initialization; this change ensures that any runtime modifications to the timestep are correctly propagated through the simulation pipeline, including contact systems, animators, and time integrators. New unit and simulation tests have been added to verify that changes to dt are correctly reflected in the simulation results. I have no feedback to provide as the review comments were purely explanatory and did not identify any issues.