[18.0][FIX] stock_secondary_unit: Keep the count exact across the whole route

[carlosdauden]

Partner sells fish by weight but some orders are placed by piece
count instead (e.g. "40 pieces" for 40 dinner guests) - the count must
survive exactly through every hop of the route and every backorder,
never be recomputed from the weight actually processed.

Adds a new "secondary_priority" dependency_type (the weight is still
estimated from the count on entry, like "dependent", but the count is
never itself recomputed back from the weight, like "independent") and
makes every quantity-touching path respect it for both this type and
the existing "independent" one, gated behind a shared
COUNT_PRESERVING_DEPENDENCY_TYPES constant:

• _prepare_move_split_vals(): the backorder created on a partial
  validation gets exactly demand-minus-done pieces, never a
  weight-proportional estimate; the "done" move's own count is synced
  to what was actually processed instead of staying at the stale
  original demand. Uses _write() + invalidate_recordset() to avoid
  retriggering the primary-quantity compute and clobbering the just-set
  real weight with a factor-based guess.
• StockMoveLine._compute_quantity(): once a line's count has been
  measured, a later write to a dependency no longer overwrites it -
  "secondary_priority" needs this same protection "independent" already
  had, since on a move LINE the target field is what was actually
  counted, not a demand estimate.
• StockRule._push_prepare_move_copy_values() (new): a route mixing
  pull and push rules (e.g. pick_ship) creates its second leg via
  copy() only after the first leg validates - core copies the DONE
  quantity forward, never the demand, so the pushed move must do the
  same for the secondary unit instead of losing the count to
  copy=False and falling back to a weight-derived guess.
• _merge_moves_fields(): sums the count across merging moves (e.g. a
  backorder's pushed leg merging into an existing move) instead of
  silently keeping whichever move happened to survive the merge.
• secondary_uom_qty_done (new stored field): core has both a demand
  (product_uom_qty) and a done aggregate (quantity, computed from
  move_line_ids.quantity) on stock.move: the secondary unit only ever
  had the demand half. Mirrors the done half the same way, so the
  above no longer need to re-derive "how many pieces were actually
  done" inline every time.

Tests cover the backorder split for both dependency types, the
move-line protection, and the pick_ship push/merge scenario with and
without a backorder.

Depends on:

• [18.0][IMP] product_secondary_unit: Add "secondary_priority" dependency type product-attribute#2398

@Tecnativa

ping @sergio-teruel @carlos-lopez-tecnativa @CarlosRoca13

[carlos-lopez-tecnativa]

Please add the external PR to test-requirements.txt in a temporary commit so it can be tested correctly on Runboat.

[carlosdauden]

Changes done