Fix twin matrix size mismatch between addtwins and merging methods

[smasongarrison]

Two issues caused the sparse matrices from the two mz_method paths to differ structurally:

1. The merging method iterated over mz_pairs (row indices) but treated the values as IDs. Changed to iterate over mz_id_pairs which holds the actual pedigree IDs. Currently masked because test data has sequential IDs matching row positions, but would break with non-sequential IDs.

2. The different computational paths (column-merge-before-tcrossprod vs pedigree-modify-then-post-copy) leave different explicit zeros in the sparse matrix. Added Matrix::drop0() before returning to normalize the sparse representation so both methods produce structurally identical results.

https://claude.ai/code/session_01PXbgV7bdA9sg7St6xBbg3T

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