Hollow Clock V

The popular [Hollow Clock 4](https://www.thingiverse.com/thing:5636482/) is now thoroughly improved and refined.

* The ring is much thinner and neat (only 7mm) with indexes.
* There are no screw heads on the exterior (except the center of the hands).
* USB-C power inlet is placed on the back of the base.
* Ratchet mechanism makes the clock reliable against the slippage (out of step) of the weak stepper motor.
* Accuracy of the clock is much improved by the crystal oscillator of RP2040 based microcontroller.
* Easy time adjustment method (pull out the display ring, adjust, then insert to the base) is carried over from the previous model, but the ring is now securely fixed.
* Totally redesigned to exploit the texture of build plate and layer lines. Frontal surfaces of the clock are textured by the build plate.

June 5, 2024 : "base-front" and "base-rear" are slightly improved for reliable USB-C connection.

July 13, 2024 : Back cover for "Seeed Studi