[codex] Fix density reinitialization callback

[svchb]

Summary

• store the particle system passed to DensityReinitializationCallback and reinitialize only that system
• trigger dt-based reinitialization when the requested interval is reached, not only after it is exceeded
• mark the integrator state as modified after density and pressure fields are changed
• add focused unit coverage for show output, dt triggering, and selected-system reinitialization

Root Cause

The callback constructor accepted a specific particle system, but the affect path called the global reinit_density!(vu_ode, semi) helper, which loops over every system in the semidiscretization. The time-based condition also used a strict > comparison, delaying callbacks that land exactly on the configured interval.

Validation

• julia --project=. -e 'using Test, TrixiParticles; include("test/callbacks/density_reinit.jl")'\n- julia --project=. -e 'using Test, TrixiParticles, Printf; include("test/callbacks/callbacks.jl")'

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.95%. Comparing base (7e43937) to head (fcf5838).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1181      +/-   ##
==========================================
+ Coverage   89.92%   89.95%   +0.02%     
==========================================
  Files         132      133       +1     
  Lines       10434    10449      +15     
==========================================
+ Hits         9383     9399      +16     
+ Misses       1051     1050       -1     

Flag	Coverage Δ
total	89.95% <100.00%> (+0.02%)	⬆️
unit	70.74% <84.21%> (+0.22%)	⬆️

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[efaulhaber]

Doesn't work because the system might be replaced in the semidiscretization.