Verifies encode_varint_raw_48bit and calc_uint64_48bit_force for the
8 corner-case MAC addresses requested in review:
00:00:00:00:00:00, 11:00:00:00:00:00, 00:AA:00:00:00:00,
00:00:BB:00:00:00, 00:00:00:CC:00:00, 00:00:00:00:DD:00,
00:00:00:00:00:EE, FF:FF:FF:FF:FF:FF
For each value the test asserts byte-identical output to the reference
encode_varint_raw_64 loop, the expected encoded byte length, agreement
with calc_uint64_48bit_force, and round-trip through a generic varint
decoder. Adds a 100-value deterministic-random sample across the full
48-bit space for additional coverage.
