Belgian companies hiring flexi-workers must transmit the salary data of
each validated flexi payslip to the NSSO via the Flexi@Work eGov service
(flexi-counter on mycareer.be). A declaration must be sent every time a
flexi payslip is created.

New `l10n.be.flexi.at.work` model handles the full FLXWAGE flow: XML
generation from payslip data, XSD validation against the quarterly ONSS
schema, and batch submission. Supports submission, modification, and
cancellation operations.

The ONSS batch submission infrastructure (XML generation, schema
caching,
signature/go file handling, state machine) has been extracted into a new
abstract `l10n.be.onss.batch.declaration` model that both DMFA and
Flexi@Work now share — DMFA previously owned all of this inline.

`_get_flxwage_amounts()` on the payslip splits the flexi gross into the
two eGov financial-element buckets (remuneration 0001001000 / bonus
0002001000), pro-rating the flexi holiday pay (FLEXI_PECULE) across both
in proportion to their respective bases.

An action on validated flexi payslips triggers declaration creation, and
a warning in the payslip list alerts users when flexi payslips still
need
to be declared.

task-6180365