Add composite/formative construct support to modsem_pi

[Kss2k]

• Fully-composite interaction terms (all components defined with <) are now modelled as composites too (XZ < x1z1 + ... instead of =~)
• Residual covariances are skipped for composite interaction terms by default; toggle with the new composite.int.res.cov argument
• Mean-structure constraint (constrained.prod.mean) is skipped for composite interaction terms, matching UCA's intent for latent products
• method="ca" now errors when any composite variable appears in an interaction term (mixed or fully composite)
• Linear composites not involved in any interaction are passed through unchanged for all PI methods
• Rename test_formative.R -> test_composites_da.R; add test_composites_pi.R with 27 tests for the new behaviour

[chatgpt-codex-connector]

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