Get Exercise Counts By Category Path

Return { "Grade / Chapter / Thematic": count } for every distinct category_path in the library (Legacy included). The key format is segments.join(" / ") so the caller can build both chapter-level and thematic-level badges in one round-trip:

counts["Terminale / Probabilités"] // chapter-level counts["Terminale / Probabilités / Loi binomiale"] // thematic-level

Chapter-level counts are rolled up by the caller from the thematic entries (or read directly when an exercise is stored at chapter depth).