Refactor active models to use modular includes (move off monolithic *_shared.stan)

[WestonVoglesonger]

Per @pearsonca's call in #58: the modular inst/stan/transformed_parameters/*.stan files are the intended building blocks for assembling different model variants (e.g. constant_phi vs B-spline; cnty_sch vs cnty_sch_linear). The active models currently #include the monolithic model/{v6,stateonly}_shared.stan introduced in #57, which short-circuits that modularity.

Scope

• Rewrite inst/stan/impute_school_coverage_process_{stateonly,v6}.stan to #include from inst/stan/transformed_parameters/ (and inst/stan/model/'s individual files) instead of the monolithic *_shared.stan files.
• Decide what to do with model/v6_shared.stan and model/stateonly_shared.stan after the refactor — delete if unreferenced, keep as one-call wrappers if useful elsewhere.
• Confirm both active models still compile + R CMD check passes on the matrix.
• Truly-unreachable files (in any model variant) can be pruned in the same PR.

Not urgent

Not on the path to first CRAN acceptance — won't block #31. File it so the architectural direction Carl clarified in #58 isn't lost.

Refs #57 (introduced the monolithic shared files), #58 (the question that surfaced this).