Every funded day the allocator places already carries a machine-readable
`reason` code — the constants block in allocation_service.py even says it
exists "so the UI can explain itself" — but no user has ever seen one.
The calendar showed what was decided, never why, which is the one question
this product exists to answer.
Clicking a planned day now also raises the existing calendar toast with a
plain-Swedish (and English) sentence for that date and parent: a pinned day
says the user chose it, an allocator-placed day explains the placement, and
anything unrecognised — or a session where no proposal has landed yet —
shows nothing rather than a raw code.
The reason is deliberately not persisted: it is stripped at apply and
`planned_leave_day` has no column for it, so the source is the live
in-memory proposal, merged last-writer-wins from Calendar's own sync
response and PlanSyncProvider's latest. Before the first sync of a session
there is simply no explanation to show.
Also extends the backend's field-parity walk to ProposedDay, which it did
not cover — the reason could have been dropped from the wire without a
single test failing.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
