mirror = (upstream_port ^ (mac[0]<<8 | mac[5])); clamped to >=1024.
Same server+upstream always yields the same local port so the user
knows where to connect without checking the UI each time. Falls back
to OS-assigned port if the deterministic one is already in use.
Server reads its hostname via gethostname(2) and includes it as a
length-prefixed UTF-8 string at the start of the MANIFEST payload.
Client displays 'Server: <hostname> — <MAC>' in a label above the
upstream list. Both sides updated for the new MANIFEST format:
[hostname_len:1][hostname:N][entries...]
Protocol version unchanged (still 1); MANIFEST payload layout change
is backward-incompatible but both sides ship together.