Add GaussianFullCov: self-contained full-covariance Gaussian estimator

[cweniger]

Summary

• Introduces GaussianFullCov(StepwiseEstimator) in falcon/estimators/gaussian_fullcov.py — a clean, self-contained replacement for the Gaussian factory + LossBasedEstimator indirection
• Gamma is resolved once at __init__ into _proposal_gamma and _posterior_gamma, eliminating the implicit (1+gamma)/gamma transformation that was buried in sample_posterior
• gaussian.py and LossBasedEstimator are left untouched — GaussianFullCov coexists alongside the old path until it is validated and the old code is removed
• Smoke test examples 04_gaussian and 05_linear_regression switched to falcon.estimators.GaussianFullCov

Motivation

The old Gaussian factory returned a generic LossBasedEstimator configured with GaussianPosterior. This split the full-covariance Gaussian logic across three places (gaussian.py, stepwise_base.py::LossBasedEstimator, and embedded_posterior.py), with gamma handling that was particularly hard to follow. GaussianFullCov consolidates everything into one class whose name reflects what it specifically does.

Test plan

• Run pytest tests/test_examples_smoke.py -k 04_gaussian to confirm the example still trains end-to-end with GaussianFullCov
• Run pytest tests/test_examples_smoke.py -k 05_linear_regression likewise
• Confirm from falcon.estimators import GaussianFullCov works and Gaussian still resolves as before

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 165 lines in your changes missing coverage. Please review.
✅ Project coverage is 9.13%. Comparing base (f2b9c87) to head (b4a4f34).

Files with missing lines	Patch %	Lines
falcon/estimators/gaussian_fullcov.py	0.00%	164 Missing ⚠️
falcon/estimators/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main     #73      +/-   ##
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- Coverage   9.50%   9.13%   -0.38%     
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  Files         32      33       +1     
  Lines       3975    4140     +165     
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  Hits         378     378              
- Misses      3597    3762     +165     

Flag	Coverage Δ
unit	9.13% <0.00%> (-0.38%)	⬇️

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