Rethink internal Complexity API

Specifically, some of the functions are being passed `input_dir` and `output_dir` params.

It might make more sense to pass them things like `json_dir`, `templates_dir`, `css_dir`, etc.

Start by locating the internal functions that accept input_dir and output_dir, then trace their callers to understand the current API. Done means agreeing on the directory-specific parameter design and updating the affected functions and callers consistently; the issue names no files or tests.

Written by the indexing model from the issue text.