Return the delta feed for the world named by your API-Key header: every
element created/updated (`op: "upsert"`, full body) or deleted (`op:
"delete"`) since `since_cursor`, in apply order, in pages of `limit`
(default 25, max 1000). Entries carry FULL element bodies — a default page
stays inside any client's token budget; measured worlds ran ~1.5-2k chars
PER ENTRY, so raise `limit` only if you truly need bigger pages, and prefer
paging with the cursor.
Omit `since_cursor` (or pass "") to start from the beginning of the feed —
for a big world that is a multi-page walk, not one response. Pass the
returned `cursor` back as `since_cursor` to get the next page (or, later,
only what changed since); treat the cursor as OPAQUE. `has_more` true means
page again with the new cursor. Requires a READ or WRITE API-Key. This
mirrors `GET /api/v2/changes`.