A shift is two signed ledger events, no mutable table: new shift_open event
type + existing shift_z_report. The operator is the logged-in user (carried in
event identity); a shift is open iff their latest shift event is a shift_open.
ShiftService: close sums payment events in [start,end] by tender (cash/card, by
payment time), appends the signed shift_z_report (totals/counts/window), and
prints via a new generic PrinterDevice.printReport(title, lines) (Rongta ESC/POS
text) to a booth-receipt printer. Print is best-effort — a failed print does not
undo the signed close.
Routes (cashier/operator/admin): GET /api/shift/current, POST /api/shift/open
(409 if open), POST /api/shift/close (409 if none). Web ShiftControl in the
shell (non-readonly): Start/End + Z-report totals.
Verified: open -> double-open 409 -> payments (cash+card; one outside the window
excluded) -> close totals correct + signed + printed -> close-again 409 ->
re-open ok; readonly 403; verifyChain ok.
