Add Hexpad macropad submission

[Antoncarlens-code]

Submitting Hexpad — a hex-shaped 6-key macropad with a rotary encoder
(Seeed XIAO RP2040, direct-wired, no diodes). Folder: hackpads/Hexpad-macropad/.

Check off the following to let the reviewers know you've done everything:

• I HAVE RAN DRC ON MY DESIGN WITH ZERO ERRORS (Silkscreen errors are okay)
• I HAVE BRANDED ALL MY PARTS (PCB, CASE) WITH MY HACKPAD'S NAME
• I HAVE MADE SURE MY 3D PRINTED PARTS DO NOT REQUIRE SUPPORTS (Text branding overhang is okay)
• I HAVE MADE SURE TO INCLUDE TOLERANCES (0.25mm)
• MY HACKPAD'S PCB IS UNDER 100mm x 100mm, I DO NOT HAVE MORE THAN 16X SWITCHES, 2X ROTARY ENCODERS, 1X OLED, 16X LEDS.
• IN MY FOLDER, THERE IS A README.MD FILE THAT CONTAINS THE FOLLOWING INFORMATION:
  • Full render including all the parts
  • A few sentences about the inspiration and challenges
  • BOM
  • Photos of the Schematic, PCB, and case

Notes for reviewers:

• PCB DRC: 0 errors, 0 unconnected (only silkscreen + courtyard warnings — the XIAO is back-mounted under the keys). Schematic ERC: 0 violations.
• 2-layer board, 76 × 74 mm; 6 MX switches + 1 EC11 encoder.
• "Hexpad" on the PCB silkscreen and engraved into the top plate.