[Keyboard] Add Wolf Singa Kohaku Solder, Solder RGB, and Hotswap

[amorino]

Description

The Wolf Kohaku by SingaKBD ships with QMK and VIA support and is actively used by the community, but the keyboard definitions were never added to the QMK repo. This PR adds three variants, built from the official VIA JSONs and tested on actual PCBs, some of which were provided by SingaKBD.

• wolf/singa/kohaku_solder — Solder, no RGB
• wolf/singa/kohaku_solder_rgb — Solder with RGB underglow
• wolf/singa/kohaku_hotswap — Hotswap, no RGB

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• N/A

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[drashna]

I've only marked one of the keyboard versions, but applies to all