fix: handle module buttons on either master half

[frankebel]

Disclaimer: I used an LLM to debug.

Fix module push buttons (knob press of the encoder-rev2 module)
being dropped when the button module's half is the slave.

• rules.mk:
  Compile halcyon_buttons.c in every build.
  The button may only exist on the slave half.
  The master side needs to interpret the keycode.

• halcyon_buttons.c:
  Select the button table from the event's matrix row
  (row < ROWS_PER_HAND = button lives on the left half).
  The old is_keyboard_left() is only correct if the left side is the master.

Tested on Halcyon Corne.

Closes #19.

[veypatch]

Thank you very much for the PR, that was indeed an oversight.