Use "résultat" and "traiter" in the French task vocabulary instead of "complétion"

#3613729: Split the overloaded result vocabulary into the outcome and the completion payload introduced "complétion" into the French translations for the
completion payload and its scope, and the completer field had already picked it
up. In French IT usage "complétion" first reads as autocomplétion (shell
or editor completion), not as the act of finishing a task, so the labels land
wrong for a French-speaking workflow author.

French does not need to mirror the English split anyway: the English rename
existed because "result" and "outcome" are synonyms, while French already
distinguishes "issue" (the outcome an outgoing flow routes on) from "résultat"
(what the step wrote). So the machine keys stay as they are and only the
translations change:

• Payload variable and Completion scope become "Variable de
  résultat" and "Portée du résultat", with the descriptions that mention them.
• The completer strings drop the word too: Completer becomes
  "Terminé par", and "Enregistrer dans une variable l'auteur de la complétion"
  becomes "Enregistrer dans une variable la personne qui a terminé l'étape".

18 strings across the core translation and five submodules
(orchestra_eca, orchestra_inbox, orchestra_interaction,
orchestra_interaction_webform, orchestra_views). No code, config or schema
change.

Second wording in the same family: "terminer une tâche" is a literal
rendering of complete, where French workflow and back-office software
says traiter ("dossiers à traiter", "traité par", "en cours de
traitement"). So Completer becomes "Traité par", "Tâche %label
terminée" becomes "Tâche %label traitée", and "La date d'achèvement de
l'élément" becomes "La date de traitement de l'élément". A process is not
traité, it ends, so the instance states keep "terminé".

Splitting those two senses needs a little code, the only code here: a task
and an instance both read Completed in English with no translation
context, so one po entry covered both. The work item's status label now carries
['context' => 'Work item status'] and gets its own entry, while
the plain msgid stays with the instance. The work item's completed
timestamp field, a third user of the same string, is relabelled
Completed on, which is clearer English for a date column.