Previously: `SERIAL=$(date +%Y%m%d)01` — same-day re-runs produced the
same serial. HE polled, saw no change, never pulled the update.
Now: scan zones-prepared/ for the highest `YYYYMMDDNN` matching today's
date and increment the NN counter. First run of the day starts at NN=01.
Caps at NN=99 with a clear error message (set SERIAL manually if you
genuinely need >99 changes per day).
`SERIAL=<value> make prep` still overrides the auto-detection, useful
for forcing a specific serial during recovery or for testing.
Verified end-to-end on dell01: prep bumped 2026051601 → 2026051602,
CoreDNS auto-reload picked it up within 30s, all queried zones serve
the new serial. HE will pull on its next refresh poll (SOA refresh
= 3600s, so worst case 1 hour).
Ryan Malloy
87eaa27c4c