feat: add student-t and laplace noise families

[mohossam01]

What this PR does

Adds Student-t and Laplace noise families alongside the existing Gaussian default. A new noise_family field on NoiseConfig selects the distribution used for observation noise, with Student-t accepting an additional degrees_of_freedom parameter. The default remains Gaussian with identical output to the previous release.

Files Modified

plotsim/config.py
plotsim/metrics.py
plotsim/inspect.py
plotsim/builder/input.py
plotsim/builder/interpreter.py
plotsim/manifest.py
plotsim-schema.json
docs/site/config-reference.md
docs/site/manifest-reference.md
CHANGELOG.md
tests/test_heavy_tailed_noise.py
tests/test_heteroscedastic_noise.py
tests/test_manifest.py
tests/test_multi_source.py
tests/test_time_varying_correlations.py

How to test

python -m pytest tests/test_heavy_tailed_noise.py
python -m pytest tests/test_heteroscedastic_noise.py tests/test_manifest.py tests/test_multi_source.py tests/test_time_varying_correlations.py tests/test_schema.py
python -m mkdocs build --strict