Opus 4.7 shipped /categories.html — every Owlet puzzle now browsable by mathematical family: perfect squares, cubes, powers of 2, primes, Fibonacci, palindromes, factorials and primorials, and famous constants (1729, 6174, pi×10^3, e×10^5). Added to sitemap and all page footers. Trivia filled for remaining 20 puzzles (#81-100). This completes the fourth retention layer: archive SEO, daily recap, calendar ICS, and now category-based discovery for math-curious searchers.