[@mantine/schedule] Fix O(n²) date re-parsing in WeekView event positioning

[eh-bartosz-lewandowski]

What & why

getWeekPositionedEvents assigns overlapping events to columns in an O(n²) pass, and on every pairwise comparison it re-parsed dates with dayjs — isEventsOverlap parsed all four start/end strings, and columnHasConflict recomputed each event's all-day flag by calling isAllDayEvent across every weekday. At ~1,500 events that is ~1.3M dayjs parses ≈ 9.6s to lay out one week, and it degrades quadratically.

Change

Parse each date once instead of on every comparison:

• Cache each event's { start, end } timestamps in a WeakMap in is-events-overlap (also benefits DayView / ResourcesDayView, which use the same util).
• Compute the all-day flag once when an event is placed into a column and store it as ColumnEvent { event, allDay }, so columnHasConflict reads the precomputed flag instead of recomputing isAllDayEvent.

This is a pure memoization / precompute — layout output is unchanged. The stored per-event all-day flag is equivalent to the original per-comparison recompute: isAllDayEvent is true only on the event's own start day, which is always within the event's covered days.

Result

Positioning 1,500 events: ~9,600ms → ~125ms (~77×); a typical week lays out in a few ms.

Notes

The WeakMap assumes events are immutable value objects (matching the library's callback-based update model). Mutating an existing event's start/end in place while reusing the same object reference across positioning passes would serve cached timestamps — not a concern for normal usage.

Tests

Updated find-available-column unit tests for the new column shape.