Per-field undo in a record store

A greenhouse keeps notes on each plant bed in a record store: a record per bed ("bed3"), fields like "crop" or "watered". Staff keep mistyping things, so every field gets its own undo button.

Build UndoStore with:

• set(key, field, value) -> None: write the field (creating the record if needed). Every call counts as a change, even if the value is the same as before.
• get(key, field) -> str | None: the current value, or None if the record or field is missing.
• delete(key, field) -> bool: remove the field and return True, or return False if it wasn't there. A delete that returns False is not a change.
• undo(key, field) -> bool: revert the most recent change to that one field that hasn't been undone yet, and return True. The field goes back to the value it had just before that change, or disappears if it didn't exist then. Return False (changing nothing) if there is nothing left to undo for that field. Undo can be repeated all the way back to the field's very first change. Undo only affects the named field.
• fields(key) -> list[str]: the record's current field names, sorted; [] for a missing record.
• count() -> int: how many records have at least one field right now.

There is no redo: an undone change is gone for good.

s = UndoStore()
s.set("bed3", "crop", "kale")
s.set("bed3", "crop", "chard")
s.set("bed3", "watered", "mon")
s.delete("bed3", "crop")      # True
s.get("bed3", "crop")         # None
s.undo("bed3", "crop")        # True   (the delete is undone)
s.get("bed3", "crop")         # "chard"
s.undo("bed3", "crop")        # True
s.get("bed3", "crop")         # "kale"
s.undo("bed3", "crop")        # True   (the very first set is undone)
s.get("bed3", "crop")         # None
s.undo("bed3", "crop")        # False  (nothing left)
s.fields("bed3")              # ["watered"]
s.undo("bed3", "watered")     # True
s.count()                     # 0

count() is called a lot, so keep it O(1): don't scan all records.