Consolidate duplicated path-get and flatten implementations

[thorwhalen]

The codebase has three overlapping path-get implementations — path_get, _path_get, and chain_get — and two unrelated flattens: trans.flatten vs paths.flatten_dict. This is a concrete, self-contained slice of the broader "centralize path access" theme (Discussion #21).

Goal: pick one canonical path_get/path_set, route the others to it (with deprecation shims), and disambiguate the two flattens by name/role.

See misc/docs/dol_architecture_map.md §7 and §11.