fix: encode sACN discovery universes in network order

[iibaranov-IG]

Fixes #142.

The discovery PDU copied uint16_t bytes directly from an array of sacn_output_universe structures. Besides retaining host byte order, this reads later values with the wrong stride because each source entry contains timestamp and mark fields.

This serializes each universe field individually with htobe16. The new native regression uses universes 1, 256, and 0x1234 and checks the exact payload bytes 00 01 01 00 12 34, covering both byte order and structure stride.

Validation:

• make test passes on macOS;
• the focused test and sACN shared backend compile and run on Ubuntu 24.04;
• BCL reproduces the baseline failure and verifies the candidate repair;
• all 81 BCL infrastructure tests pass.

The complete optional-backend build was not run because the local host lacks Lua development headers; the sACN backend itself builds successfully.