``hr_timesheet_sheet`` works on every analytic line of a period at once, while
``project_timesheet_holidays`` protects the analytic lines it generates for validated
time off against any modification or deletion. Together they make any timesheet sheet
that spans a day with validated time off unmanageable: opening, recomputing, creating or
saving such a sheet raises a ``UserError`` and the employee cannot record their hours.

This bridge module makes time off lines inert with respect to the bulk operations of the
sheet while keeping them visible inside it, so that the week is shown complete with its
time off hours included. Two different code paths are involved and both are handled: the
bulk ``write`` that links or unlinks ``sheet_id``, and the ``Command.DELETE`` the web
client emits for the lines that leave the set cached by the onchange, which ends up in a
real ``unlink()``. In the second case the time off line is detached from the sheet
(``sheet_id = False``) instead of being deleted; the time off request and the employee
balance are never touched.

The protection itself is not relaxed: the hours, date, project, task, employee and
description of a time off line still cannot be changed from a timesheet sheet, the line
still cannot be deleted by hand and new timesheets still cannot be created on a time off
task. Only a write that touches nothing but the neutral link fields is let through.

The module also closes a pre-existing hole in ``hr_timesheet_sheet``, whose
``_check_can_update_timesheet()`` returned ``True`` for any recordset without
``sheet_id`` and therefore silently disabled the core protection for every time off line
not linked to a sheet.