Add idx_exclude to MarginalizingTimingModel

[vhaasteren]

Summary

• Adds an idx_exclude parameter to MarginalizingTimingModel, mirroring the existing API on TimingModel.
• Excluded columns are dropped before basis construction (including the SVD path), so the marginalized timing-model matrix only spans the remaining linear parameters.
• Adds unit tests for default behavior, column exclusion, and SVD + exclusion, plus a likelihood smoke test.

This is a small API parity change. It lets callers marginalize over a subset of timing-model columns while handling the excluded ones elsewhere — for example, when a nonlinear delay term samples some parameters and the remaining linear timing parameters are marginalized analytically.

Test plan

• pytest tests/test_gp_signals.py::TestGPSignals::test_marginalizing_timing_model_idx_exclude
• pytest tests/test_likelihood.py::TestLikelihood::test_like_marginalizing_tm_idx_exclude
• Full Enterprise test suite (if CI is available)