When an operator types a quantity on a transfer line, that quantity is the one
that was physically handled: a bag of apples ordered as 1 kg that weighs
0.87 kg on the scale.

Odoo, however, treats a quantity below the demand as an incomplete
*reservation*. The move goes back to ``partially_available`` and
``_action_assign`` tops it up again to the demanded quantity, either from the
``Procurement: run scheduler`` cron or from ``_trigger_assign``, which runs
every time another transfer of the same product is validated. A few hours
later the weighed 0.87 kg is back to 1 kg. Raising the quantity is never
reverted, because the move stays ``assigned`` and no longer matches those
domains.

This module makes a hand-typed quantity final:

* Editing a quantity from the interface marks the move as **picked**, which is
  what keeps ``_action_assign`` and ``_free_reservation`` away from it.
* When the typed quantity is below the demand, the demand is lowered to match,
  so the move stays ``assigned`` and no backorder is asked for a quantity that
  will never be completed.
* Transfers still validate every line that carries a quantity, picked by hand
  or not, so freezing one line never pushes the untouched ones to a backorder.