Share one bound activity schema between the source and store contracts #31

graph-source-contract (#25) and graph-store-contract (#26) each define boundActivitySchema (the model's activity with source required and equal to its id) by unwrapping graph-schema's modelSchema, and the store's items.upsert input mirrors a source sync page's participants, activities, transitions and overlay. Both were written in parallel, so the two definitions are identical in intent but separate in code.

Once both PRs are on main, move the bound activity schema (and its type) into graph-schema beside modelSchema, have both contracts import it, and check that a sync page's items pass boundItemsSchema unchanged. Deferred only because neither PR can depend on the other while both are open.