reach-edu-hub onboards to @dididecks/shell — and a deck-collections pattern worth lifting

reach-edu-hub becomes the shell's fourth consumer, joining chroma, humain, and lossless. The onboarding also produced two reusable ideas the shell doesn't have yet: deck collections (a menu layer above single-deck convergence) and a token-driven deck-primitives system — both prototyped in reach as lift candidates. The pass also surfaced one concrete shell limitation: the slot registry is keyed by variant slug alone, so sibling decks can't reuse a variant name.