Merchant-hosted endpoint called by SKUU. The customer API root is the
reviewed prefix immediately before the fixed /products path.

The response is an authoritative baseline. Every variant must occur exactly
once across the completed traversal and must include exactly one inventory
level for every agreed active physical location plus the merchant's one
SKUU_Network location. Zero quantities must be explicit. At initial
acceptance the network-location quantity is zero; after writable go-live it
is the last absolute level written by SKUU.

SKUU requests 100 products by default, accepts at most the requested limit,
converts a string or integer next_cursor to a query string for the next
request, rejects empty or repeated cursors, and stops when next_cursor is
absent or null. The client has a 2,000-page runaway-cursor backstop. A terminal
page may be empty, but a complete initial catalogue may not be empty.

SKUU validates the complete remote traversal before accepting a new
baseline. After a successful non-empty traversal, merchant variants absent
from the complete feed are archived and zeroed. An incomplete or invalid
traversal leaves the existing baseline unchanged.