Camera card: find by extension, delete folders

A camera writes photos, videos and sidecar files onto its memory card in nested folders such as /DCIM/2025_06_01/IMG_0001.jpg. You're writing the little in-memory model the camera's menu uses.

Implement CameraCard() with:

• save(path) -> None: store a file at path, creating any missing parent folders. Saving to a path that already holds a file changes nothing (it is still one file).
• find(folder, ext) -> list[str]: the full paths of every file anywhere under folder (at any depth) whose name ends with "." + ext, sorted. ext is given without the dot and matching is case-sensitive. A folder that doesn't exist (or a path that is a file) gives [].
• remove(path) -> int: delete the file or the whole folder at path and return how many files were deleted. Deleting a folder takes everything inside it. The folder that contained it stays, even if it is now empty. If nothing exists at path, return 0 and change nothing. remove("/") empties the card (the root itself stays).

card = CameraCard()
card.save("/DCIM/day1/a.jpg")
card.save("/DCIM/day1/a.xmp")
card.save("/DCIM/day2/b.jpg")
card.save("/c.jpg")
card.find("/", "jpg")          # ["/DCIM/day1/a.jpg", "/DCIM/day2/b.jpg", "/c.jpg"]
card.find("/DCIM/day1", "xmp") # ["/DCIM/day1/a.xmp"]
card.remove("/DCIM/day1")      # 2
card.find("/DCIM", "jpg")      # ["/DCIM/day2/b.jpg"]
card.remove("/DCIM/day1")      # 0   (already gone)

Paths are absolute, have no trailing slash (except the root "/"), and names are non-empty. Nobody saves a file where a folder is, or inside a file. A name like "jpg" (no dot) does not end with ".jpg". Up to a few thousand operations.