Check lot quantities

modules/purchase_trade/docs/business/lots-and-quantities.en.md

@@ -227,6 +227,10 @@ Invariants to preserve after every update:
   where `lot_p = virtual lot`, whether they are matched to a `lot_s` or not.
 - For a sale virtual lot, the second invariant sums all `lot.qt` lines where
   `lot_s = virtual lot`, whether they are linked to a `lot_p` or not.
+- A zero `lot.qt` row is accepted if it still keeps at least one `lot_p` or
+  `lot_s` link: it is then the memory of a consumed forecast.
+- A `lot.qt` row without both `lot_p` and `lot_s` is blocked by
+  `lot.qt.validate`.
 - The technical guard is centralized in
   `lot.lot.assert_lines_quantity_consistency()` and must be called at the end
   of flows that modify lots, matching, transport, weighing, or the contractual