(In)Compatible Observables and Density Matrix

Round-14 hunt: picked; source: named-course long-video search (r13 queries, single-hit channels, second look); channel screens (sellers/exam/re-upload/devotional/vendor/events), provenance check, vendor/seller/devotional recheck and quality tier C + tier-C depth check passed; strict per-title check + final strict pass; oEmbed verified.