fix: refresh Langevin sigma on friction changes

[galjos]

Summary

• Recompute LangevinThermostat::_sigma inside setFriction().
• Add a regression test matching the existing target-temperature sigma recomputation test.

Root cause

The Langevin noise amplitude is derived from friction and target temperature. setTargetTemperature() already refreshed _sigma, but setFriction() left the dependent state stale.

Validation

• cmake --build build-stochastic-rescale-ci --target testThermostat --parallel 4
• build-stochastic-rescale-ci/tests/src/thermostat/testThermostat

Reference

• Langevin dynamics random/noise term depends on friction and temperature: https://en.wikipedia.org/wiki/Langevin_dynamics

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.12%. Comparing base (1d02915) to head (4a740c5).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #273   +/-   ##
=======================================
  Coverage   88.12%   88.12%           
=======================================
  Files         283      283           
  Lines       10874    10875    +1     
  Branches     3376     3376           
=======================================
+ Hits         9583     9584    +1     
  Misses       1251     1251           
  Partials       40       40           

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[pq-perf-bot]

⚡ Performance (instruction count) — ✅ no regressions

per-benchmark breakdown

benchmark	base Ir	PR Ir	Δ
bondedForces	41.46M	41.46M	-0.00%	✅
boxTransforms	10.67M	10.67M	+0.00%	✅
constraints	11.38M	11.38M	+0.00%	✅
coulombKernel	5.80M	5.80M	+0.00%	✅
forceKernel	15.33M	15.33M	-0.00%	✅
integrator	32.17M	32.17M	+0.00%	✅
kinetics	9.53M	9.53M	+0.00%	✅
linearAlgebra	2.08M	2.08M	+0.00%	✅
nonCoulombPairs	5.48M	5.48M	-0.00%	✅
shiftVector	5.81M	5.81M	+0.00%	✅
virial	11.63M	11.63M	+0.00%	✅

_{Deterministic callgrind instruction counts vs the base branch; gated at ±2%. Not wall-clock.}