Add Metakey Tenet 70 Support

[connorgallopo]

Description

Adds support for the Metakey Tenet 70 — a transformable (regular / southpaw) 70% keyboard.

This being my first contribution, and first time trying to reverse an existing keyboard... This was an absolute bear. The reversible ribbon connected 10 key daughter board (which also has the OLED screen) as well as the OLED functionality itself took me way longer than i'd like to admit to get working.

The OLED screen does not retain the factory screens (Time, Typing Cat, WPM, etc) and right now there is a keybind that on keyboard allows you to edit the text displayed and that is saved to memory. It ships default with "TENET QMK".

The other interesting thing is I believe this firmware has massively increased performance of this keyboard. I would always have rollover issues, issues with missed key presses (on random keys), other various quirks. I always had the sneaking suspicion it was how they manage the date/time, the wpm tracker, and the other animations. Cant say for certain, but I can say that i have had 0 issues since flashing with this!

Also a big win as MetaKey has pretty much abandoned the software suite that controlled the device and has not shipped any fw updates since launch.

Not sure how many there are out there, but I think this is a big win for Tenet users.

Hardware:

• MCU: Holtek HT32F52352 (Cortex-M0+, 128KB flash, 16KB SRAM)
• Bootloader: stock Hexcore IAP (app slot at 0x04000), so BOOTLOADER = custom with a PROGRAM_CMD that calls an external flasher
• OLED: SSD1326 256x32 mono, driven directly from the main MCU over I2C0
• Per-key RGB: 76 LEDs driven by two Holtek sub-MCUs over a shared SPI bus, wired through a custom WS2812 driver
• Custom 5×18 matrix, with two pins shared with USB/SWD handled at scan time

Everything lives under keyboards/metakey/tenet/ and ships a default keymap. VIA support will be submitted separately to qmk_userspace_via, and the VIA definition to the-via/keyboards, per the PR checklist.

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).