refactor(ggm): share active<->full index traversal via for_each_active_full_pair

[MaartenMarsman]

What

get_full_vectorized_parameters (scatters active theta into the full zero-padded vector) and get_active_inv_mass (gathers the full-dim inverse mass back into the active layout) open-coded the same per-column index arithmetic in inverse directions — the same theta_offsets/full_theta_offsets/included_indices/psi_offset/full_psi_offset walk, written twice.

Adds GraphConstraintStructure::for_each_active_full_pair(fn), which visits each active entry once as (active_index, full_index) — the d_q free off-diagonal entries then psi_q, per column. The scatter and gather are now one-line lambdas over it:

cs.for_each_active_full_pair([&](size_t a, size_t f){ full(f)   = theta_(a);    }); // scatter
cs.for_each_active_full_pair([&](size_t a, size_t f){ active(a) = inv_mass_(f); }); // gather

The two are now provably exact inverses (same index pairs), which the old duplicated loops only implied.

Why this is safe

Identical (active, full) index pairs in identical order; the assignments are unchanged. psi_offset(q) == theta_offsets[q] + d_q, so the helper's psi pair matches the old scatter's theta_(active_offset + col.d_q).

Verified bitwise: update_method="nuts" GGM fits with diagonal mass adaptation (which is what exercises these two functions — scatter feeds Welford during warmup, gather returns the adapted mass), edge-selection (sparse active dim, exercises included_indices) and no-edge. identical(main, branch) == TRUE. Harness: dev/bitwise_ggm_nuts.R.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.73%. Comparing base (07b1137) to head (e9b99ae).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #139      +/-   ##
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+ Coverage   87.71%   87.73%   +0.01%     
==========================================
  Files          87       87              
  Lines       12867    12861       -6     
==========================================
- Hits        11286    11283       -3     
+ Misses       1581     1578       -3     

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