Discover Project Haystack tag/def taxonomy via the defs op

Steps

1. POST to /haystack/defs with an empty grid to fetch all defs, or with filter/limit columns for a subset
2. Use a filter string such as "def==^point" to fetch a specific def dict
3. Read each response row as a def dict, including def, lib, is, mandatory, and doc columns
4. Use the related libs, ops, and filetypes ops (same request/response convention) to explore just those def subsets