Round 22: OpenGraph + Twitter cards, pricing tier tests, PDP slug smoke tests

Site-wide social-share metadata: OG and Twitter summary_large_image cards cascade to every marketing page. 14 pure-function unit tests cover the trade-discount logic unified in rounds 19/21. 12 live-stack smoke tests cover the PDP slug routing added in round 18 — malformed slugs, non-existent refs, path traversal attempts.