refactor(mixed): RAII guard for proposed-state OMRF marginal evaluation

[MaartenMarsman]

What

The three continuous Kyy moves — update_pairwise_effects_continuous_offdiag, update_pairwise_effects_continuous_diag, and update_edge_indicator_continuous — each evaluate the OMRF marginal log-likelihood under the proposed (Kyy', Σ') by temporarily setting pairwise_effects_continuous_, covariance_continuous_, and marginal_interactions_, summing log_marginal_omrf, then restoring the accepted state. This was open-coded as three near-identical save / mutate / recompute / restore blocks (offdiag and edge-indicator byte-identical; diag saved a single element).

This replaces the manual save/restore with a file-local ProposedContinuousState RAII guard that snapshots the three fields on construction and restores them on scope exit. Each call site becomes a { } block: construct guard → set proposed values → recompute_marginal_interactions() → sum marginals → (scope exit restores).

Removes the manual-restore footgun — any future field touched inside the block can no longer leak past it by a forgotten restore.

Why this is safe

Same fields mutated, same recompute_marginal_interactions(), same values restored. Restore order among the three independent fields is irrelevant. The diag case now snapshots the whole pairwise_effects_continuous_ matrix rather than the single (i,i) element, but only (i,i) is ever changed in the block, so the restored matrix is identical.

Verified bitwise: adaptive-Metropolis mixed-MRF raw draws (main + pairwise + indicator) are digest-identical pre/post across edge-sel delta=0, no-edge delta=0, and no-edge delta=1. identical(main, branch) == TRUE. Harness: dev/bitwise_mixed_logdet_ratio.R.

[codecov]

Codecov Report

❌ Patch coverage is 53.33333% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.72%. Comparing base (44efd57) to head (ec419cb).

Files with missing lines	Patch %	Lines
src/models/mixed/mixed_mrf_metropolis.cpp	53.33%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
+ Coverage   87.61%   87.72%   +0.10%     
==========================================
  Files          87       87              
  Lines       12891    12883       -8     
==========================================
+ Hits        11295    11301       +6     
+ Misses       1596     1582      -14     

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