[ADD] survey_xlsx_expand_multiple_choice, survey_xlsx_extra_fields

survey_xlsx_expand_multiple_choice: expand multiple_choice questions into
one Oui/Non column per option, and matrix questions into one column per
row (value = selected option). Relies on the extension hooks added to
survey_xlsx.

survey_xlsx_extra_fields: bridge (auto_install) with survey_extra_fields
that excludes 'file' question types from the XLSX export.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

survey_xlsx_expand_multiple_choice/readme/CONTRIBUTORS.rst

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+* `Elabore <https://www.elabore.coop>`_
+
+  * Quentin Mondot

survey_xlsx_expand_multiple_choice/readme/DESCRIPTION.rst

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+This module improves the **Survey Results XLSX export** provided by
+``survey_xlsx`` for questions that can hold several answers.
+
+By default such questions are exported as a single column containing every
+selected value joined together, which is hard to analyse in a spreadsheet.
+This module splits them into dedicated columns:
+
+* **Multiple choice** questions (*multiple answers allowed*): one column per
+  possible answer, with ``Oui`` / ``Non`` as value.
+* **Matrix** questions: one column per matrix row, with the selected option
+  as value.
+
+.. warning::
+
+   This module relies on report extension hooks that are **not part of the
+   standard** ``survey_xlsx`` yet. They are introduced by this pull request:
+
+   https://github.com/elabore-coop/survey/pull/1
+
+   You must run a ``survey_xlsx`` that includes these hooks (the PR branch,
+   until it is merged upstream). Installed against a plain ``survey_xlsx``,
+   this module installs without error but the export **silently falls back**
+   to the default one-column-per-question behaviour.

survey_xlsx_expand_multiple_choice/readme/USAGE.rst

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+Export the results of a survey as usual:
+
+#. Go to *Surveys* and open a survey.
+#. Print the *Survey Results XLSX* report.
+
+In the generated spreadsheet:
+
+* A multiple choice question ``Favorite colors`` with options *Red*, *Green*
+  and *Blue* produces three columns ``Favorite colors / Red``,
+  ``Favorite colors / Green`` and ``Favorite colors / Blue``, each containing
+  ``Oui`` or ``Non``.
+* A matrix question ``Satisfaction`` with rows *Dashboards* and *Customer
+  relationship* produces two columns ``Satisfaction / Dashboards`` and
+  ``Satisfaction / Customer relationship``, each containing the selected
+  option (e.g. *Not satisfied at all*).
+
+Other question types keep their standard single-column export.